diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 50b418ec9..23559eb35 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -2531,12 +2531,15 @@ UniValue lockunspent(const JSONRPCRequest& request)
 
     RPCTypeCheckArgument(request.params[1], UniValue::VARR);
 
-    UniValue outputs = request.params[1].get_array();
-    for (unsigned int idx = 0; idx < outputs.size(); idx++) {
-        const UniValue& output = outputs[idx];
-        if (!output.isObject())
-            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected object");
-        const UniValue& o = output.get_obj();
+    const UniValue& output_params = request.params[1];
+
+    // Create and validate the COutPoints first.
+
+    std::vector<COutPoint> outputs;
+    outputs.reserve(output_params.size());
+
+    for (unsigned int idx = 0; idx < output_params.size(); idx++) {
+        const UniValue& o = output_params[idx].get_obj();
 
         RPCTypeCheckObj(o,
             {
@@ -2544,20 +2547,50 @@ UniValue lockunspent(const JSONRPCRequest& request)
                 {"vout", UniValueType(UniValue::VNUM)},
             });
 
-        std::string txid = find_value(o, "txid").get_str();
-        if (!IsHex(txid))
+        const std::string& txid = find_value(o, "txid").get_str();
+        if (!IsHex(txid)) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected hex txid");
+        }
 
-        int nOutput = find_value(o, "vout").get_int();
-        if (nOutput < 0)
+        const int nOutput = find_value(o, "vout").get_int();
+        if (nOutput < 0) {
             throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout must be positive");
+        }
 
-        COutPoint outpt(uint256S(txid), nOutput);
+        const COutPoint outpt(uint256S(txid), nOutput);
 
-        if (fUnlock)
-            pwallet->UnlockCoin(outpt);
-        else
-            pwallet->LockCoin(outpt);
+        const auto it = pwallet->mapWallet.find(outpt.hash);
+        if (it == pwallet->mapWallet.end()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, unknown transaction");
+        }
+
+        const CWalletTx& trans = it->second;
+
+        if (outpt.n >= trans.tx->vout.size()) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, vout index out of bounds");
+        }
+
+        if (pwallet->IsSpent(outpt.hash, outpt.n)) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected unspent output");
+        }
+
+        const bool is_locked = pwallet->IsLockedCoin(outpt.hash, outpt.n);
+
+        if (fUnlock && !is_locked) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, expected locked output");
+        }
+
+        if (!fUnlock && is_locked) {
+            throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid parameter, output already locked");
+        }
+
+        outputs.push_back(outpt);
+    }
+
+    // Atomically set (un)locked status for the outputs.
+    for (const COutPoint& outpt : outputs) {
+        if (fUnlock) pwallet->UnlockCoin(outpt);
+        else pwallet->LockCoin(outpt);
     }
 
     return true;
diff --git a/test/functional/wallet.py b/test/functional/wallet.py
index 9d8ae5035..db60df18e 100755
--- a/test/functional/wallet.py
+++ b/test/functional/wallet.py
@@ -100,11 +100,19 @@ class WalletTest(BitcoinTestFramework):
         # Exercise locking of unspent outputs
         unspent_0 = self.nodes[2].listunspent()[0]
         unspent_0 = {"txid": unspent_0["txid"], "vout": unspent_0["vout"]}
+        assert_raises_rpc_error(-8, "Invalid parameter, expected locked output", self.nodes[2].lockunspent, True, [unspent_0])
         self.nodes[2].lockunspent(False, [unspent_0])
+        assert_raises_rpc_error(-8, "Invalid parameter, output already locked", self.nodes[2].lockunspent, False, [unspent_0])
         assert_raises_rpc_error(-4, "Insufficient funds", self.nodes[2].sendtoaddress, self.nodes[2].getnewaddress(), 20)
         assert_equal([unspent_0], self.nodes[2].listlockunspent())
         self.nodes[2].lockunspent(True, [unspent_0])
         assert_equal(len(self.nodes[2].listlockunspent()), 0)
+        assert_raises_rpc_error(-8, "Invalid parameter, unknown transaction",
+                              self.nodes[2].lockunspent, False,
+                              [{"txid": "0000000000000000000000000000000000", "vout": 0}])
+        assert_raises_rpc_error(-8, "Invalid parameter, vout index out of bounds",
+                              self.nodes[2].lockunspent, False,
+                              [{"txid": unspent_0["txid"], "vout": 999}])
 
         # Have node1 generate 100 blocks (so node0 can recover the fee)
         self.nodes[1].generate(100)
@@ -143,6 +151,10 @@ class WalletTest(BitcoinTestFramework):
         assert_equal(self.nodes[2].getbalance(), 94)
         assert_equal(self.nodes[2].getbalance("from1"), 94-21)
 
+        # Verify that a spent output cannot be locked anymore
+        spent_0 = {"txid": node0utxos[0]["txid"], "vout": node0utxos[0]["vout"]}
+        assert_raises_rpc_error(-8, "Invalid parameter, expected unspent output", self.nodes[0].lockunspent, False, [spent_0])
+
         # Send 10 BTC normal
         address = self.nodes[0].getnewaddress("test")
         fee_per_byte = Decimal('0.001') / 1000