diff --git a/api/api_full.go b/api/api_full.go
index 6e6796476..6b3dfa6fd 100644
--- a/api/api_full.go
+++ b/api/api_full.go
@@ -41,6 +41,7 @@ type FullNode interface {
 	SyncState(context.Context) (*SyncState, error)
 	SyncSubmitBlock(ctx context.Context, blk *types.BlockMsg) error
 	SyncIncomingBlocks(ctx context.Context) (<-chan *types.BlockHeader, error)
+	SyncMarkBad(ctx context.Context, bcid cid.Cid) error
 
 	// messages
 	MpoolPending(context.Context, *types.TipSet) ([]*types.SignedMessage, error)
diff --git a/api/apistruct/struct.go b/api/apistruct/struct.go
index 7a95f318c..1f7c97f95 100644
--- a/api/apistruct/struct.go
+++ b/api/apistruct/struct.go
@@ -58,6 +58,7 @@ type FullNodeStruct struct {
 		SyncState          func(context.Context) (*api.SyncState, error)                `perm:"read"`
 		SyncSubmitBlock    func(ctx context.Context, blk *types.BlockMsg) error         `perm:"write"`
 		SyncIncomingBlocks func(ctx context.Context) (<-chan *types.BlockHeader, error) `perm:"read"`
+		SyncMarkBad        func(ctx context.Context, bcid cid.Cid) error                `perm:"write"`
 
 		MpoolPending     func(context.Context, *types.TipSet) ([]*types.SignedMessage, error) `perm:"read"`
 		MpoolPush        func(context.Context, *types.SignedMessage) error                    `perm:"write"`
@@ -360,6 +361,10 @@ func (c *FullNodeStruct) SyncIncomingBlocks(ctx context.Context) (<-chan *types.
 	return c.Internal.SyncIncomingBlocks(ctx)
 }
 
+func (c *FullNodeStruct) SyncMarkBad(ctx context.Context, bcid cid.Cid) error {
+	return c.Internal.SyncMarkBad(ctx, bcid)
+}
+
 func (c *FullNodeStruct) StateMinerSectors(ctx context.Context, addr address.Address, ts *types.TipSet) ([]*api.ChainSectorInfo, error) {
 	return c.Internal.StateMinerSectors(ctx, addr, ts)
 }
diff --git a/chain/sync.go b/chain/sync.go
index 19d83df27..9065bc099 100644
--- a/chain/sync.go
+++ b/chain/sync.go
@@ -1181,3 +1181,7 @@ func (syncer *Syncer) State() []SyncerState {
 	}
 	return out
 }
+
+func (syncer *Syncer) MarkBad(blk cid.Cid) {
+	syncer.bad.Add(blk)
+}
diff --git a/cli/sync.go b/cli/sync.go
index 1b051769a..d3d425967 100644
--- a/cli/sync.go
+++ b/cli/sync.go
@@ -19,6 +19,7 @@ var syncCmd = &cli.Command{
 	Subcommands: []*cli.Command{
 		syncStatusCmd,
 		syncWaitCmd,
+		syncMarkBadCmd,
 	},
 }
 
@@ -90,6 +91,30 @@ var syncWaitCmd = &cli.Command{
 	},
 }
 
+var syncMarkBadCmd = &cli.Command{
+	Name:  "mark-bad",
+	Usage: "Mark the given block as bad, will prevent syncing to a chain that contains it",
+	Action: func(cctx *cli.Context) error {
+		napi, closer, err := GetFullNodeAPI(cctx)
+		if err != nil {
+			return err
+		}
+		defer closer()
+		ctx := ReqContext(cctx)
+
+		if !cctx.Args().Present() {
+			return fmt.Errorf("must specify block cid to mark")
+		}
+
+		bcid, err := cid.Decode(cctx.Args().First())
+		if err != nil {
+			return fmt.Errorf("failed to decode input as a cid: %s", err)
+		}
+
+		return napi.SyncMarkBad(ctx, bcid)
+	},
+}
+
 func SyncWait(ctx context.Context, napi api.FullNode) error {
 	for {
 		state, err := napi.SyncState(ctx)
diff --git a/node/impl/full/sync.go b/node/impl/full/sync.go
index 0ebdad536..250837ab3 100644
--- a/node/impl/full/sync.go
+++ b/node/impl/full/sync.go
@@ -6,6 +6,8 @@ import (
 	"github.com/filecoin-project/lotus/api"
 	"github.com/filecoin-project/lotus/chain"
 	"github.com/filecoin-project/lotus/chain/types"
+	cid "github.com/ipfs/go-cid"
+	"github.com/prometheus/common/log"
 
 	pubsub "github.com/libp2p/go-libp2p-pubsub"
 	"go.uber.org/fx"
@@ -80,3 +82,9 @@ func (a *SyncAPI) SyncSubmitBlock(ctx context.Context, blk *types.BlockMsg) erro
 func (a *SyncAPI) SyncIncomingBlocks(ctx context.Context) (<-chan *types.BlockHeader, error) {
 	return a.Syncer.IncomingBlocks(ctx)
 }
+
+func (a *SyncAPI) SyncMarkBad(ctx context.Context, bcid cid.Cid) error {
+	log.Warnf("Marking block %s as bad", bcid)
+	a.Syncer.MarkBad(bcid)
+	return nil
+}