diff --git a/src/Makefile.test.include b/src/Makefile.test.include
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -51,6 +51,7 @@
   test/blockstatus_tests.cpp \
   test/bloom_tests.cpp \
   test/bswap_tests.cpp \
+  test/byteswap_tests.cpp \
   test/cashaddr_tests.cpp \
   test/cashaddrenc_tests.cpp \
   test/checkdatasig_tests.cpp \
diff --git a/src/qt/test/compattests.cpp b/src/qt/test/compattests.cpp
--- a/src/qt/test/compattests.cpp
+++ b/src/qt/test/compattests.cpp
@@ -16,7 +16,7 @@
 #include <compat/byteswap.h>
 
 void CompatTests::bswapTests() {
-    // Sibling in bitcoin/src/test/bswap_tests.cpp
+    // Sibling in bitcoin/src/test/byteswap_tests.cpp
     uint16_t u1 = 0x1234;
     uint32_t u2 = 0x56789abc;
     uint64_t u3 = 0xdef0123456789abc;
diff --git a/src/script/interpreter.cpp b/src/script/interpreter.cpp
--- a/src/script/interpreter.cpp
+++ b/src/script/interpreter.cpp
@@ -1300,6 +1300,22 @@
                         }
                     } break;
 
+                    case OP_BSWAP: {
+                        if (!(flags & SCRIPT_ENABLE_OP_BSWAP)) {
+                            return set_error(serror, ScriptError::BAD_OPCODE);
+                        }
+
+                        // (in -- out)
+                        if (stack.size() < 1) {
+                            return set_error(
+                                serror, ScriptError::INVALID_STACK_OPERATION);
+                        }
+
+                        valtype &data = stacktop(-1);
+
+                        std::reverse(data.begin(), data.end());
+                    } break;
+
                     default:
                         return set_error(serror, ScriptError::BAD_OPCODE);
                 }
diff --git a/src/script/script_flags.h b/src/script/script_flags.h
--- a/src/script/script_flags.h
+++ b/src/script/script_flags.h
@@ -105,6 +105,9 @@
     // Whether to allow new OP_CHECKMULTISIG logic to trigger. (new multisig
     // logic verifies faster, and only allows Schnorr signatures)
     SCRIPT_ENABLE_SCHNORR_MULTISIG = (1U << 21),
+
+    // Whether the new OP_BSWAP opcode can be used.
+    SCRIPT_ENABLE_OP_BSWAP = (1U << 22),
 };
 
 #endif // BITCOIN_SCRIPT_SCRIPT_FLAGS_H
diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -73,6 +73,7 @@
 		blockstatus_tests.cpp
 		bloom_tests.cpp
 		bswap_tests.cpp
+		byteswap_tests.cpp
 		cashaddr_tests.cpp
 		cashaddrenc_tests.cpp
 		checkdatasig_tests.cpp
diff --git a/src/test/bswap_tests.cpp b/src/test/bswap_tests.cpp
--- a/src/test/bswap_tests.cpp
+++ b/src/test/bswap_tests.cpp
@@ -1,26 +1,126 @@
-// Copyright (c) 2016 The Bitcoin Core developers
+// Copyright (c) 2018 The Bitcoin developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <compat/byteswap.h>
+#include <policy/policy.h>
+#include <script/interpreter.h>
 
+#include <test/lcg.h>
 #include <test/test_bitcoin.h>
 
 #include <boost/test/unit_test.hpp>
 
+#include <array>
+#include <bitset>
+
+typedef std::vector<uint8_t> valtype;
+typedef std::vector<valtype> stacktype;
+
 BOOST_FIXTURE_TEST_SUITE(bswap_tests, BasicTestingSetup)
 
-BOOST_AUTO_TEST_CASE(bswap_tests) {
-    // Sibling in bitcoin/src/qt/test/compattests.cpp
-    uint16_t u1 = 0x1234;
-    uint32_t u2 = 0x56789abc;
-    uint64_t u3 = 0xdef0123456789abc;
-    uint16_t e1 = 0x3412;
-    uint32_t e2 = 0xbc9a7856;
-    uint64_t e3 = 0xbc9a78563412f0de;
-    BOOST_CHECK(bswap_16(u1) == e1);
-    BOOST_CHECK(bswap_32(u2) == e2);
-    BOOST_CHECK(bswap_64(u3) == e3);
+std::array<uint32_t, 3> flagset{{0, STANDARD_SCRIPT_VERIFY_FLAGS}};
+
+static void CheckError(uint32_t flags, const stacktype &original_stack,
+                       const CScript &script, ScriptError expected) {
+    BaseSignatureChecker sigchecker;
+    ScriptError err = ScriptError::OK;
+    stacktype stack{original_stack};
+    bool r = EvalScript(stack, script, flags, sigchecker, &err);
+    BOOST_CHECK(!r);
+    BOOST_CHECK(err == expected);
+}
+
+static void CheckPass(uint32_t flags, const stacktype &original_stack,
+                      const CScript &script, const stacktype &expected) {
+    BaseSignatureChecker sigchecker;
+    ScriptError err = ScriptError::OK;
+    stacktype stack{original_stack};
+    bool r = EvalScript(stack, script, flags, sigchecker, &err);
+    BOOST_CHECK(r);
+    BOOST_CHECK(err == ScriptError::OK);
+    BOOST_CHECK(stack == expected);
+}
+
+/**
+ * General utility functions to check for script passing/failing.
+ */
+static void CheckErrorForAllFlags(const stacktype &original_stack,
+                                  const CScript &script, ScriptError expected) {
+    for (uint32_t flags : flagset) {
+        // The script generates the proper error regardless of whether or not
+        // SCRIPT_ENABLE_OP_BSWAP flag is passed.
+        CheckError(flags & ~SCRIPT_ENABLE_OP_BSWAP, original_stack, script,
+                   expected);
+        CheckError(flags | SCRIPT_ENABLE_OP_BSWAP, original_stack, script,
+                   expected);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(bswap_test) {
+    // Opcode not enabled.
+    CheckError(STANDARD_SCRIPT_VERIFY_FLAGS & ~SCRIPT_ENABLE_OP_BSWAP, {},
+               CScript() << OP_BSWAP, ScriptError::BAD_OPCODE);
+
+    // Empty stack.
+    CheckError(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP, {},
+               CScript() << OP_BSWAP, ScriptError::INVALID_STACK_OPERATION);
+
+    // Manual tests.
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP, {{}},
+              CScript() << OP_BSWAP, {{}});
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP, {{99}},
+              CScript() << OP_BSWAP, {{99}});
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+              {{0xde, 0xad}}, CScript() << OP_BSWAP, {{0xad, 0xde}});
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+              {{0xde, 0xad, 0xa1}}, CScript() << OP_BSWAP,
+              {{0xa1, 0xad, 0xde}});
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+              {{0xde, 0xad, 0xbe, 0xef}}, CScript() << OP_BSWAP,
+              {{0xef, 0xbe, 0xad, 0xde}});
+
+    // Generated tests, sizes 0 to 520 that shoud succeed
+    // + 20 that should fail.
+    for (size_t datasize = 0; datasize <= 520 + 20; ++datasize) {
+        valtype data;
+        data.reserve(datasize);
+        for (uint8_t item = 0; item < (uint8_t)datasize; ++item) {
+            data.push_back(item);
+        }
+        if (datasize <= 520) {
+            valtype reversed_data;
+            reversed_data.reserve(datasize);
+            for (uint8_t item = (uint8_t)datasize - 1; item >= 0; --item) {
+                reversed_data.push_back(item);
+            }
+            CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+                      {data}, CScript() << OP_BSWAP, {reversed_data});
+        } else {
+            CheckErrorForAllFlags({data}, CScript() << OP_BSWAP,
+                                  ScriptError::PUSH_SIZE);
+        }
+    }
+
+    // More tests.
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+              {{0x12, 0x34, 0x56}, {0x56, 0x34, 0x12}},
+              CScript() << OP_BSWAP << OP_EQUALVERIFY, {});
+
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+              {{0x02, 0x04, 0x06, 0x08, 0x0A, 0x0C}},
+              CScript() << OP_DUP << OP_BSWAP << OP_BSWAP << OP_EQUALVERIFY,
+              {});
+
+    // Verify palindrome.
+    CheckPass(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+              {{0x01, 0x02, 0x03, 0x02, 0x01}},
+              CScript() << OP_DUP << OP_BSWAP << OP_EQUALVERIFY, {});
+
+    // Verify non-palindrome fails.
+    CheckError(STANDARD_SCRIPT_VERIFY_FLAGS | SCRIPT_ENABLE_OP_BSWAP,
+               {{0x01, 0x02, 0x03, 0x02, 0x02}},
+               CScript() << OP_DUP << OP_BSWAP << OP_EQUALVERIFY,
+               ScriptError::EQUALVERIFY);
 }
 
 BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/test/bswap_tests.cpp b/src/test/byteswap_tests.cpp
copy from src/test/bswap_tests.cpp
copy to src/test/byteswap_tests.cpp
--- a/src/test/bswap_tests.cpp
+++ b/src/test/byteswap_tests.cpp
@@ -8,9 +8,9 @@
 
 #include <boost/test/unit_test.hpp>
 
-BOOST_FIXTURE_TEST_SUITE(bswap_tests, BasicTestingSetup)
+BOOST_FIXTURE_TEST_SUITE(byteswap_tests, BasicTestingSetup)
 
-BOOST_AUTO_TEST_CASE(bswap_tests) {
+BOOST_AUTO_TEST_CASE(byteswap_tests) {
     // Sibling in bitcoin/src/qt/test/compattests.cpp
     uint16_t u1 = 0x1234;
     uint32_t u2 = 0x56789abc;
diff --git a/src/test/data/script_tests.json b/src/test/data/script_tests.json
--- a/src/test/data/script_tests.json
+++ b/src/test/data/script_tests.json
@@ -254,8 +254,7 @@
 
 ["0", "IF CHECKDATASIG ELSE 1 ENDIF", "P2SH,STRICTENC", "OK"],
 ["0", "IF CHECKDATASIGVERIFY ELSE 1 ENDIF", "P2SH,STRICTENC", "OK"],
-["0", "IF 0xbc ELSE 1 ENDIF", "P2SH,STRICTENC", "OK", "opcodes >= FIRST_UNDEFINED_OP_VALUE invalid if executed"],
-["0", "IF 0xbd ELSE 1 ENDIF", "P2SH,STRICTENC", "OK"],
+["0", "IF 0xbd ELSE 1 ENDIF", "P2SH,STRICTENC", "OK", "opcodes >= FIRST_UNDEFINED_OP_VALUE invalid if executed"],
 ["0", "IF 0xbe ELSE 1 ENDIF", "P2SH,STRICTENC", "OK"],
 ["0", "IF 0xbf ELSE 1 ENDIF", "P2SH,STRICTENC", "OK"],
 ["0", "IF 0xc0 ELSE 1 ENDIF", "P2SH,STRICTENC", "OK"],
@@ -1241,8 +1240,7 @@
  "P2SH,DISCOURAGE_UPGRADABLE_NOPS", "DISCOURAGE_UPGRADABLE_NOPS", "Discouraged NOP10 in redeemScript"],
 
 ["0x50","1", "P2SH,STRICTENC", "BAD_OPCODE", "opcode 0x50 is reserved"],
-["1", "IF 0xbc ELSE 1 ENDIF", "P2SH,STRICTENC", "BAD_OPCODE", "opcodes >= FIRST_UNDEFINED_OP_VALUE invalid if executed"],
-["1", "IF 0xbd ELSE 1 ENDIF", "P2SH,STRICTENC", "BAD_OPCODE"],
+["1", "IF 0xbd ELSE 1 ENDIF", "P2SH,STRICTENC", "BAD_OPCODE", "opcodes >= FIRST_UNDEFINED_OP_VALUE invalid if executed"],
 ["1", "IF 0xbe ELSE 1 ENDIF", "P2SH,STRICTENC", "BAD_OPCODE"],
 ["1", "IF 0xbf ELSE 1 ENDIF", "P2SH,STRICTENC", "BAD_OPCODE"],
 ["1", "IF 0xc0 ELSE 1 ENDIF", "P2SH,STRICTENC", "BAD_OPCODE"],
@@ -1362,7 +1360,7 @@
 ["1","RESERVED", "P2SH,STRICTENC", "BAD_OPCODE", "OP_RESERVED is reserved"],
 ["1","RESERVED1", "P2SH,STRICTENC", "BAD_OPCODE", "OP_RESERVED1 is reserved"],
 ["1","RESERVED2", "P2SH,STRICTENC", "BAD_OPCODE", "OP_RESERVED2 is reserved"],
-["1","0xbc", "P2SH,STRICTENC", "BAD_OPCODE", "0xbc == FIRST_UNDEFINED_OP_VALUE"],
+["1","0xbd", "P2SH,STRICTENC", "BAD_OPCODE", "0xbd == FIRST_UNDEFINED_OP_VALUE"],
 
 ["2147483648", "1ADD 1", "P2SH,STRICTENC", "UNKNOWN_ERROR", "We cannot do math on 5-byte integers"],
 ["2147483648", "NEGATE 1", "P2SH,STRICTENC", "UNKNOWN_ERROR", "We cannot do math on 5-byte integers"],
diff --git a/src/test/scriptflags.cpp b/src/test/scriptflags.cpp
--- a/src/test/scriptflags.cpp
+++ b/src/test/scriptflags.cpp
@@ -33,6 +33,7 @@
     {"CHECKDATASIG", SCRIPT_VERIFY_CHECKDATASIG_SIGOPS},
     {"DISALLOW_SEGWIT_RECOVERY", SCRIPT_DISALLOW_SEGWIT_RECOVERY},
     {"SCHNORR_MULTISIG", SCRIPT_ENABLE_SCHNORR_MULTISIG},
+    {"BSWAP", SCRIPT_ENABLE_OP_BSWAP},
 };
 
 uint32_t ParseScriptFlags(std::string strFlags) {