diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -2397,12 +2397,58 @@
         while (hasValidAncestor && pindexTest && pindexTest != pindexFork) {
             assert(pindexTest->nChainTx || pindexTest->nHeight == 0);
 
+            // If this is a parked chain, but it has enough PoW, clear the park
+            // state.
+            bool fParkedChain = pindexTest->nStatus.isOnParkedChain();
+            if (fParkedChain && gArgs.GetBoolArg("-parkdeepreorg", true)) {
+                const CBlockIndex *pindexTip = chainActive.Tip();
+
+                // During initialization, pindexTip and/or pindexFork may be
+                // null. In this case, we just ignore the fact that the chain is
+                // parked.
+                if (!pindexTip || !pindexFork) {
+                    UnparkBlock(pindexTest);
+                    continue;
+                }
+
+                // A parked chain can be unparked if it has twice as much PoW
+                // accumulated as the main chain has since the fork block.
+                CBlockIndex const *pindexExtraPow = pindexTip;
+                arith_uint256 requiredWork = pindexTip->nChainWork;
+                switch (pindexTip->nHeight - pindexFork->nHeight) {
+                    // Limit the penality for depth 1, 2 and 3 to half a block
+                    // worth of work to ensure we don't fork accidentaly.
+                    case 3:
+                    case 2:
+                        pindexExtraPow = pindexExtraPow->pprev;
+                    // FALLTHROUGH
+                    case 1: {
+                        const arith_uint256 deltaWork =
+                            pindexExtraPow->nChainWork - pindexFork->nChainWork;
+                        requiredWork += (deltaWork >> 1);
+                        break;
+                    }
+                    default:
+                        requiredWork +=
+                            pindexExtraPow->nChainWork - pindexFork->nChainWork;
+                        break;
+                }
+
+                if (pindexNew->nChainWork > requiredWork) {
+                    // We have enough, clear the parked state.
+                    LogPrintf("Unpark block %s as its chain has accumulated "
+                              "enough PoW.\n",
+                              pindexTest->GetBlockHash().ToString());
+                    fParkedChain = false;
+                    UnparkBlock(pindexTest);
+                }
+            }
+
             // Pruned nodes may have entries in setBlockIndexCandidates for
             // which block files have been deleted. Remove those as candidates
             // for the most work chain if we come across them; we can't switch
             // to a chain unless we have all the non-active-chain parent blocks.
             bool fInvalidChain = pindexTest->nStatus.isInvalid();
-            bool fParkedChain = pindexTest->nStatus.isOnParkedChain();
             bool fMissingData = !pindexTest->nStatus.hasData();
             if (!(fInvalidChain || fParkedChain || fMissingData)) {
                 // The current block is acceptable, move to the parent, up to
@@ -2721,6 +2767,10 @@
             nBlockReverseSequenceId--;
         }
 
+        // In case this was parked, unpark it.
+        UnparkBlock(pindex);
+
+        // Make sure it is added to the candidate list if apropriate.
         if (pindex->IsValid(BlockValidity::TRANSACTIONS) && pindex->nChainTx) {
             setBlockIndexCandidates.insert(pindex);
             PruneBlockIndexCandidates();
@@ -3621,6 +3671,20 @@
                      block.GetHash().ToString());
     }
 
+    // If this is a deep reorg (a regorg of more than one block), preemptively
+    // mark the chain as parked. If it has enough work, it'll unpark
+    // automatically. We mark the block as parked at the very last minute so we
+    // can make sure everything is ready to be reorged if needed.
+    if (gArgs.GetBoolArg("-parkdeepreorg", true)) {
+        const CBlockIndex *pindexFork = chainActive.FindFork(pindex);
+        if (pindexFork && pindexFork->nHeight + 1 < pindex->nHeight) {
+            LogPrintf("Park block %s as it would cause a deep reorg.\n",
+                      pindex->GetBlockHash().ToString());
+            pindex->nStatus = pindex->nStatus.withParked();
+            setDirtyBlockIndex.insert(pindex);
+        }
+    }
+
     // Header is valid/has work and the merkle tree is good.
     // Relay now, but if it does not build on our best tip, let the
     // SendMessages loop relay it.
diff --git a/test/functional/abc-parkedchain.py b/test/functional/abc-parkedchain.py
--- a/test/functional/abc-parkedchain.py
+++ b/test/functional/abc-parkedchain.py
@@ -6,12 +6,13 @@
 import os
 
 from test_framework.test_framework import BitcoinTestFramework
-from test_framework.util import assert_equal
+from test_framework.util import assert_equal, connect_nodes_bi, sync_blocks, wait_until
 
 
 class ParkedChainTest(BitcoinTestFramework):
     def set_test_params(self):
-        self.num_nodes = 1
+        self.num_nodes = 2
+        self.extra_args = [["-noparkdeepreorg"], []]
 
     # There should only be one chaintip, which is expected_tip
     def only_valid_tip(self, expected_tip, other_tip_status=None):
@@ -36,6 +37,8 @@
 
         # Let's park the chain.
         assert(parked_tip != tip)
+        assert(block_to_park != tip)
+        assert(block_to_park != parked_tip)
         node.parkblock(block_to_park)
         assert_equal(node.getbestblockhash(), tip)
 
@@ -122,6 +125,49 @@
         node.reconsiderblock(bad_tip)
         self.only_valid_tip(good_tip)
 
+        # First, make sure both nodes are in sync.
+        parking_node = self.nodes[1]
+        connect_nodes_bi(self.nodes, 0, 1)
+        sync_blocks(self.nodes[0:2])
+
+        assert_equal(node.getbestblockhash(), parking_node.getbestblockhash())
+
+        # Wait for node 1 to park the chain.
+        def wait_for_parked_block(block):
+            def check_block():
+                for tip in parking_node.getchaintips():
+                    if tip["hash"] == block:
+                        assert(tip["status"] != "active")
+                        return tip["status"] == "parked"
+                return False
+            wait_until(check_block)
+
+        def check_reorg_protection(depth, extra_blocks):
+            self.log.info("Test deep reorg parking, %d block deep" % depth)
+
+            # Invalidate the tip on node 0, so it doesn't follow node 1.
+            node.invalidateblock(node.getbestblockhash())
+            # Mine block to create a fork of proper depth
+            parking_node.generate(depth - 1)
+            node.generate(depth)
+            # extra block should now find themselves parked
+            for i in range(extra_blocks):
+                node.generate(1)
+                wait_for_parked_block(node.getbestblockhash())
+
+            # If we mine one more block, the node reorgs.
+            node.generate(1)
+            wait_until(lambda: parking_node.getbestblockhash()
+                       == node.getbestblockhash())
+
+        check_reorg_protection(1, 0)
+        check_reorg_protection(2, 0)
+        check_reorg_protection(3, 1)
+        check_reorg_protection(4, 4)
+        check_reorg_protection(5, 5)
+        check_reorg_protection(6, 6)
+        check_reorg_protection(100, 100)
+
 
 if __name__ == '__main__':
     ParkedChainTest().main()
diff --git a/test/functional/bip68-sequence.py b/test/functional/bip68-sequence.py
--- a/test/functional/bip68-sequence.py
+++ b/test/functional/bip68-sequence.py
@@ -26,7 +26,7 @@
 class BIP68Test(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 2
-        self.extra_args = [["-blockprioritypercentage=0"],
+        self.extra_args = [["-blockprioritypercentage=0", "-noparkdeepreorg"],
                            ["-blockprioritypercentage=0", "-acceptnonstdtxn=0"]]
 
     def run_test(self):
diff --git a/test/functional/dbcrash.py b/test/functional/dbcrash.py
--- a/test/functional/dbcrash.py
+++ b/test/functional/dbcrash.py
@@ -53,7 +53,8 @@
         # Set -rpcservertimeout=900 to reduce socket disconnects in this
         # long-running test
         self.base_args = ["-limitdescendantsize=0", "-maxmempool=0",
-                          "-rpcservertimeout=900", "-dbbatchsize=200000"]
+                          "-rpcservertimeout=900", "-dbbatchsize=200000",
+                          "-noparkdeepreorg"]
 
         # Set different crash ratios and cache sizes.  Note that not all of
         # -dbcache goes to pcoinsTip.
diff --git a/test/functional/getchaintips.py b/test/functional/getchaintips.py
--- a/test/functional/getchaintips.py
+++ b/test/functional/getchaintips.py
@@ -14,6 +14,7 @@
 class GetChainTipsTest (BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
+        self.extra_args = [["-noparkdeepreorg"], ["-noparkdeepreorg"], [], []]
 
     def run_test(self):
         tips = self.nodes[0].getchaintips()
diff --git a/test/functional/invalidateblock.py b/test/functional/invalidateblock.py
--- a/test/functional/invalidateblock.py
+++ b/test/functional/invalidateblock.py
@@ -15,6 +15,7 @@
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 3
+        self.extra_args = [["-noparkdeepreorg"], [], []]
 
     def setup_network(self):
         self.setup_nodes()
diff --git a/test/functional/listsinceblock.py b/test/functional/listsinceblock.py
--- a/test/functional/listsinceblock.py
+++ b/test/functional/listsinceblock.py
@@ -11,6 +11,7 @@
     def set_test_params(self):
         self.num_nodes = 4
         self.setup_clean_chain = True
+        self.extra_args = [["-noparkdeepreorg"], ["-noparkdeepreorg"], [], []]
 
     def run_test(self):
         '''
diff --git a/test/functional/p2p-acceptblock.py b/test/functional/p2p-acceptblock.py
--- a/test/functional/p2p-acceptblock.py
+++ b/test/functional/p2p-acceptblock.py
@@ -76,7 +76,8 @@
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 3
-        self.extra_args = [[], ["-whitelist=127.0.0.1"],
+        self.extra_args = [["-noparkdeepreorg"],
+                           ["-noparkdeepreorg", "-whitelist=127.0.0.1"],
                            ["-minimumchainwork=0x10"]]
 
     def setup_network(self):
diff --git a/test/functional/p2p-fullblocktest.py b/test/functional/p2p-fullblocktest.py
--- a/test/functional/p2p-fullblocktest.py
+++ b/test/functional/p2p-fullblocktest.py
@@ -61,7 +61,7 @@
     # Change the "outcome" variable from each TestInstance object to only do the comparison.
     def set_test_params(self):
         self.num_nodes = 1
-        self.extra_args = [['-whitelist=127.0.0.1']]
+        self.extra_args = [['-whitelist=127.0.0.1', '-noparkdeepreorg']]
         self.setup_clean_chain = True
         self.block_heights = {}
         self.coinbase_key = CECKey()
diff --git a/test/functional/preciousblock.py b/test/functional/preciousblock.py
--- a/test/functional/preciousblock.py
+++ b/test/functional/preciousblock.py
@@ -45,6 +45,8 @@
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 3
+        self.extra_args = [["-noparkdeepreorg"],
+                           ["-noparkdeepreorg"], ["-noparkdeepreorg"]]
 
     def setup_network(self):
         self.setup_nodes()
diff --git a/test/functional/pruning.py b/test/functional/pruning.py
--- a/test/functional/pruning.py
+++ b/test/functional/pruning.py
@@ -37,16 +37,21 @@
 
         # Create nodes 0 and 1 to mine.
         # Create node 2 to test pruning.
-        self.full_node_default_args = ["-maxreceivebuffer=20000", "-blockmaxsize=999000", "-checkblocks=5",
-                                       "-limitdescendantcount=100", "-limitdescendantsize=5000", "-limitancestorcount=100", "-limitancestorsize=5000"]
+        self.full_node_default_args = ["-maxreceivebuffer=20000", "-blockmaxsize=999000",
+                                       "-checkblocks=5", "-noparkdeepreorg",
+                                       "-limitdescendantcount=100", "-limitdescendantsize=5000",
+                                       "-limitancestorcount=100", "-limitancestorsize=5000"]
         # Create nodes 3 and 4 to test manual pruning (they will be re-started with manual pruning later)
         # Create nodes 5 to test wallet in prune mode, but do not connect
         self.extra_args = [self.full_node_default_args,
                            self.full_node_default_args,
-                           ["-maxreceivebuffer=20000", "-prune=550"],
-                           ["-maxreceivebuffer=20000", "-blockmaxsize=999000"],
-                           ["-maxreceivebuffer=20000", "-blockmaxsize=999000"],
-                           ["-prune=550"]]
+                           ["-maxreceivebuffer=20000",
+                               "-prune=550", "-noparkdeepreorg"],
+                           ["-maxreceivebuffer=20000",
+                               "-blockmaxsize=999000", "-noparkdeepreorg"],
+                           ["-maxreceivebuffer=20000",
+                               "-blockmaxsize=999000", "-noparkdeepreorg"],
+                           ["-prune=550", "-noparkdeepreorg"]]
 
     def setup_network(self):
         self.setup_nodes()
@@ -146,7 +151,8 @@
         # transactions (from disconnected blocks)
         self.stop_node(1)
         self.start_node(1, extra_args=[
-                        "-maxreceivebuffer=20000", "-blockmaxsize=5000", "-checkblocks=5", "-disablesafemode"])
+                        "-maxreceivebuffer=20000", "-blockmaxsize=5000", "-checkblocks=5",
+                        "-disablesafemode", "-noparkdeepreorg"])
 
         height = self.nodes[1].getblockcount()
         self.log.info("Current block height: %d" % height)
@@ -172,7 +178,8 @@
         # Reboot node1 to clear those giant tx's from mempool
         self.stop_node(1)
         self.start_node(1, extra_args=[
-                        "-maxreceivebuffer=20000", "-blockmaxsize=5000", "-checkblocks=5", "-disablesafemode"])
+                        "-maxreceivebuffer=20000", "-blockmaxsize=5000", "-checkblocks=5",
+                        "-disablesafemode", "-noparkdeepreorg"])
 
         self.log.info("Generating new longer chain of 300 more blocks")
         self.nodes[1].generate(300)
@@ -364,7 +371,7 @@
         # check that the pruning node's wallet is still in good shape
         self.log.info("Stop and start pruning node to trigger wallet rescan")
         self.stop_node(2)
-        self.start_node(2, extra_args=["-prune=550"])
+        self.start_node(2, extra_args=["-prune=550", "-noparkdeepreorg"])
         self.log.info("Success")
 
         # check that wallet loads loads successfully when restarting a pruned node after IBD.
@@ -374,7 +381,7 @@
         nds = [self.nodes[0], self.nodes[5]]
         sync_blocks(nds, wait=5, timeout=300)
         self.stop_node(5)  # stop and start to trigger rescan
-        self.start_node(5, extra_args=["-prune=550"])
+        self.start_node(5, extra_args=["-prune=550", "-noparkdeepreorg"])
         self.log.info("Success")
 
     def run_test(self):
diff --git a/test/functional/sendheaders.py b/test/functional/sendheaders.py
--- a/test/functional/sendheaders.py
+++ b/test/functional/sendheaders.py
@@ -182,6 +182,7 @@
     def set_test_params(self):
         self.setup_clean_chain = True
         self.num_nodes = 2
+        self.extra_args = [["-noparkdeepreorg"], ["-noparkdeepreorg"]]
 
     # mine count blocks and return the new tip
     def mine_blocks(self, count):
diff --git a/test/functional/txn_clone.py b/test/functional/txn_clone.py
--- a/test/functional/txn_clone.py
+++ b/test/functional/txn_clone.py
@@ -11,6 +11,7 @@
 class TxnMallTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
+        self.extra_args = [["-noparkdeepreorg"], ["-noparkdeepreorg"], [], []]
 
     def add_options(self, parser):
         parser.add_argument("--mineblock", dest="mine_block", default=False, action="store_true",
diff --git a/test/functional/txn_doublespend.py b/test/functional/txn_doublespend.py
--- a/test/functional/txn_doublespend.py
+++ b/test/functional/txn_doublespend.py
@@ -11,6 +11,7 @@
 class TxnMallTest(BitcoinTestFramework):
     def set_test_params(self):
         self.num_nodes = 4
+        self.extra_args = [["-noparkdeepreorg"], ["-noparkdeepreorg"], [], []]
 
     def add_options(self, parser):
         parser.add_argument("--mineblock", dest="mine_block", default=False, action="store_true",