diff --git a/CHANGELOG.md b/CHANGELOG.md
index b37ba401341..636eda0fae2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -74,6 +74,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * (client/server) [#18345](https://github.com/cosmos/cosmos-sdk/pull/18345) Consistently set viper prefix in client and server. It defaults for the binary name for both client and server.
 * (simulation) [#17911](https://github.com/cosmos/cosmos-sdk/pull/17911) Fix all problems with executing command `make test-sim-custom-genesis-fast` for simulation test.
 * (simulation) [#18196](https://github.com/cosmos/cosmos-sdk/pull/18196) Fix the problem of `validator set is empty after InitGenesis` in simulation test.
+* (baseapp) [#18551](https://github.com/cosmos/cosmos-sdk/pull/18551) Fix SelectTxForProposal the calculation method of tx bytes size is inconsistent with CometBFT
 
 ### API Breaking Changes
 
diff --git a/baseapp/abci_utils.go b/baseapp/abci_utils.go
index 0b570ab52a6..4cf9fa8e228 100644
--- a/baseapp/abci_utils.go
+++ b/baseapp/abci_utils.go
@@ -10,6 +10,7 @@ import (
 	cryptoenc "github.com/cometbft/cometbft/crypto/encoding"
 	cmtprotocrypto "github.com/cometbft/cometbft/proto/tendermint/crypto"
 	cmtproto "github.com/cometbft/cometbft/proto/tendermint/types"
+	cmttypes "github.com/cometbft/cometbft/types"
 	protoio "github.com/cosmos/gogoproto/io"
 	"github.com/cosmos/gogoproto/proto"
 
@@ -368,7 +369,7 @@ func (ts *defaultTxSelector) Clear() {
 }
 
 func (ts *defaultTxSelector) SelectTxForProposal(_ context.Context, maxTxBytes, maxBlockGas uint64, memTx sdk.Tx, txBz []byte) bool {
-	txSize := uint64(len(txBz))
+	txSize := uint64(cmttypes.ComputeProtoSizeForTxs([]cmttypes.Tx{txBz}))
 
 	var txGasLimit uint64
 	if memTx != nil {
diff --git a/baseapp/abci_utils_test.go b/baseapp/abci_utils_test.go
index f204250382e..bf3748101e1 100644
--- a/baseapp/abci_utils_test.go
+++ b/baseapp/abci_utils_test.go
@@ -8,6 +8,7 @@ import (
 	"github.com/cometbft/cometbft/crypto/secp256k1"
 	cmtprotocrypto "github.com/cometbft/cometbft/proto/tendermint/crypto"
 	cmtproto "github.com/cometbft/cometbft/proto/tendermint/types"
+	cmttypes "github.com/cometbft/cometbft/types"
 	dbm "github.com/cosmos/cosmos-db"
 	protoio "github.com/cosmos/gogoproto/io"
 	"github.com/cosmos/gogoproto/proto"
@@ -310,6 +311,9 @@ func (s *ABCIUtilsTestSuite) TestDefaultProposalHandler_NoOpMempoolTxSelection()
 	s.Require().NoError(err)
 	s.Require().Len(txBz, 152)
 
+	txDataSize := int(cmttypes.ComputeProtoSizeForTxs([]cmttypes.Tx{txBz}))
+	s.Require().Equal(txDataSize, 155)
+
 	testCases := map[string]struct {
 		ctx         sdk.Context
 		req         *abci.RequestPrepareProposal
@@ -331,7 +335,7 @@ func (s *ABCIUtilsTestSuite) TestDefaultProposalHandler_NoOpMempoolTxSelection()
 			}),
 			req: &abci.RequestPrepareProposal{
 				Txs:        [][]byte{txBz, txBz, txBz, txBz, txBz},
-				MaxTxBytes: 456,
+				MaxTxBytes: 465,
 			},
 			expectedTxs: 0,
 		},
@@ -339,10 +343,18 @@ func (s *ABCIUtilsTestSuite) TestDefaultProposalHandler_NoOpMempoolTxSelection()
 			ctx: s.ctx,
 			req: &abci.RequestPrepareProposal{
 				Txs:        [][]byte{txBz, txBz, txBz, txBz, txBz},
-				MaxTxBytes: 456,
+				MaxTxBytes: 465,
 			},
 			expectedTxs: 3,
 		},
+		"large max tx bytes len calculation": {
+			ctx: s.ctx,
+			req: &abci.RequestPrepareProposal{
+				Txs:        [][]byte{txBz, txBz, txBz, txBz, txBz},
+				MaxTxBytes: 456,
+			},
+			expectedTxs: 2,
+		},
 		"max gas and tx bytes": {
 			ctx: s.ctx.WithConsensusParams(cmtproto.ConsensusParams{
 				Block: &cmtproto.BlockParams{
@@ -351,7 +363,7 @@ func (s *ABCIUtilsTestSuite) TestDefaultProposalHandler_NoOpMempoolTxSelection()
 			}),
 			req: &abci.RequestPrepareProposal{
 				Txs:        [][]byte{txBz, txBz, txBz, txBz, txBz},
-				MaxTxBytes: 456,
+				MaxTxBytes: 465,
 			},
 			expectedTxs: 2,
 		},
@@ -360,7 +372,7 @@ func (s *ABCIUtilsTestSuite) TestDefaultProposalHandler_NoOpMempoolTxSelection()
 	for name, tc := range testCases {
 		s.Run(name, func() {
 			// iterate multiple times to ensure the tx selector is cleared each time
-			for i := 0; i < 5; i++ {
+			for i := 0; i < 6; i++ {
 				resp, err := handler(tc.ctx, tc.req)
 				s.Require().NoError(err)
 				s.Require().Len(resp.Txs, tc.expectedTxs)