diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -34,6 +34,8 @@
 
 #include <event2/http.h>
 
+#include <functional>
+
 static const std::string WALLET_ENDPOINT_BASE = "/wallet/";
 
 static std::string urlDecode(const std::string &urlEncoded) {
@@ -2652,7 +2654,7 @@
             "\nArguments:\n"
             "1. \"passphrase\"     (string, required) The wallet passphrase\n"
             "2. timeout            (numeric, required) The time to keep the "
-            "decryption key in seconds.\n"
+            "decryption key in seconds; capped at 100000000 (~3 years).\n"
             "\nNote:\n"
             "Issuing the walletpassphrase command while the wallet is already "
             "unlocked will set a new unlock\n"
@@ -2687,6 +2689,20 @@
     // with.
     strWalletPass = request.params[0].get_str().c_str();
 
+    // Get the timeout
+    int64_t nSleepTime = request.params[1].get_int64();
+    // Timeout cannot be negative, otherwise it will relock immediately
+    if (nSleepTime < 0) {
+        throw JSONRPCError(RPC_INVALID_PARAMETER,
+                           "Timeout cannot be negative.");
+    }
+    // Clamp timeout
+    // larger values trigger a macos/libevent bug?
+    constexpr int64_t MAX_SLEEP_TIME = 100000000;
+    if (nSleepTime > MAX_SLEEP_TIME) {
+        nSleepTime = MAX_SLEEP_TIME;
+    }
+
     if (strWalletPass.length() > 0) {
         if (!pwallet->Unlock(strWalletPass)) {
             throw JSONRPCError(
@@ -2702,10 +2718,9 @@
 
     pwallet->TopUpKeyPool();
 
-    int64_t nSleepTime = request.params[1].get_int64();
     pwallet->nRelockTime = GetTime() + nSleepTime;
     RPCRunLater(strprintf("lockwallet(%s)", pwallet->GetName()),
-                boost::bind(LockWallet, pwallet), nSleepTime);
+                std::bind(LockWallet, pwallet), nSleepTime);
 
     return NullUniValue;
 }
diff --git a/test/functional/wallet_encryption.py b/test/functional/wallet_encryption.py
--- a/test/functional/wallet_encryption.py
+++ b/test/functional/wallet_encryption.py
@@ -10,6 +10,8 @@
 from test_framework.util import (
     assert_equal,
     assert_raises_rpc_error,
+    assert_greater_than,
+    assert_greater_than_or_equal,
 )
 
 
@@ -56,7 +58,8 @@
                                 self.nodes[0].walletpassphrase, passphrase + "wrong", 10)
 
         # Test walletlock and unlocked_until values
-        self.nodes[0].setmocktime(1)
+        self.mocktime = 1
+        self.nodes[0].setmocktime(self.mocktime)
         self.nodes[0].walletpassphrase(passphrase, 84600)
         assert_equal(privkey, self.nodes[0].dumpprivkey(address))
         assert_equal(
@@ -73,6 +76,27 @@
                                 self.nodes[0].walletpassphrase, passphrase, 10)
         self.nodes[0].walletpassphrase(passphrase2, 10)
         assert_equal(privkey, self.nodes[0].dumpprivkey(address))
+        self.nodes[0].walletlock()
+
+        # Test timeout bounds
+        assert_raises_rpc_error(-8, "Timeout cannot be negative.",
+                                self.nodes[0].walletpassphrase, passphrase2, -10)
+        # Check the timeout
+        # Check a time less than the limit
+        MAX_VALUE = 100000000
+        expected_time = self.mocktime + MAX_VALUE - 600
+        self.nodes[0].walletpassphrase(passphrase2, MAX_VALUE - 600)
+        actual_time = self.nodes[0].getwalletinfo()['unlocked_until']
+        assert_greater_than_or_equal(actual_time, expected_time)
+        # 5 second buffer
+        assert_greater_than(expected_time + 5, actual_time)
+        # Check a time greater than the limit
+        expected_time = self.mocktime + MAX_VALUE - 1
+        self.nodes[0].walletpassphrase(passphrase2, MAX_VALUE + 1000)
+        actual_time = self.nodes[0].getwalletinfo()['unlocked_until']
+        assert_greater_than_or_equal(actual_time, expected_time)
+        # 5 second buffer
+        assert_greater_than(expected_time + 5, actual_time)
 
 
 if __name__ == '__main__':