diff --git a/pkg/connector/handle_message.go b/pkg/connector/handle_message.go
index 812354b..052bd40 100644
--- a/pkg/connector/handle_message.go
+++ b/pkg/connector/handle_message.go
@@ -394,6 +394,15 @@ func isLineDecryptFallbackText(text string) bool {
 // ConvertedMessage. bodyText/unwrappedText are the (decrypted) message text as
 // returned by decryptMessageBody. Shared by the live message path and backfill.
 func (lc *LineClient) convertLineMessage(ctx context.Context, portal *bridgev2.Portal, intent bridgev2.MatrixAPI, data line.Message, bodyText, unwrappedText string, decryptionFailed bool) (*bridgev2.ConvertedMessage, error) {
+	converted, err := lc.convertLineMessageContent(ctx, portal, intent, data, bodyText, unwrappedText, decryptionFailed)
+	if err != nil {
+		return nil, err
+	}
+	lc.cutOffLongMessage(ctx, converted, ContentType(data.ContentType))
+	return converted, nil
+}
+
+func (lc *LineClient) convertLineMessageContent(ctx context.Context, portal *bridgev2.Portal, intent bridgev2.MatrixAPI, data line.Message, bodyText, unwrappedText string, decryptionFailed bool) (*bridgev2.ConvertedMessage, error) {
 	decryptedBody := bodyText
 	replyRelatesTo := lc.resolveReplyRelatesTo(ctx, &data)
 
diff --git a/pkg/connector/long_message.go b/pkg/connector/long_message.go
new file mode 100644
index 0000000..bdd9436
--- /dev/null
+++ b/pkg/connector/long_message.go
@@ -0,0 +1,116 @@
+package connector
+
+import (
+	"context"
+	"encoding/json"
+	"slices"
+	"strings"
+	"unicode/utf8"
+
+	"github.com/rs/zerolog"
+	"maunium.net/go/mautrix/bridgev2"
+	"maunium.net/go/mautrix/event"
+	"maunium.net/go/mautrix/id"
+)
+
+const (
+	maxEncodedContentBytes = 40000
+	maxFallbackBodyBytes   = 500
+	longMessageCutOffText  = "\n\n[long message cut off]"
+	longMessageCutOffHTML  = "<br><em>[long message cut off]</em>"
+)
+
+func (lc *LineClient) cutOffLongMessage(ctx context.Context, converted *bridgev2.ConvertedMessage, contentType ContentType) {
+	if converted == nil {
+		return
+	}
+	var isGhost func(id.UserID) bool
+	if lc.UserLogin != nil && lc.UserLogin.Bridge != nil {
+		isGhost = lc.UserLogin.Bridge.IsGhostMXID
+	}
+	for i, part := range converted.Parts {
+		if part == nil || part.Content == nil {
+			continue
+		}
+		before := encodedContentSize(part.Content)
+		if before <= maxEncodedContentBytes {
+			continue
+		}
+		cutOffLongContent(part.Content, isGhost)
+		zerolog.Ctx(ctx).Warn().
+			Int("content_type", int(contentType)).
+			Int("part_index", i).
+			Int("size_before", before).
+			Int("size_after", encodedContentSize(part.Content)).
+			Msg("Cut off long LINE message to fit in a Matrix event")
+	}
+}
+
+func encodedContentSize(content *event.MessageEventContent) int {
+	data, err := json.Marshal(content)
+	if err != nil {
+		return len(content.Body) + len(content.FormattedBody)
+	}
+	return len(data)
+}
+
+func cutOffLongContent(content *event.MessageEventContent, isGhost func(id.UserID) bool) {
+	if content.Mentions != nil && isGhost != nil {
+		content.Mentions.UserIDs = slices.DeleteFunc(content.Mentions.UserIDs, isGhost)
+		if encodedContentSize(content) <= maxEncodedContentBytes {
+			return
+		}
+	}
+	if content.FormattedBody != "" && len(content.Body) > maxFallbackBodyBytes {
+		content.Body = truncateUTF8(content.Body, maxFallbackBodyBytes) + "[...]"
+		if encodedContentSize(content) <= maxEncodedContentBytes {
+			return
+		}
+	}
+	if content.FormattedBody != "" {
+		shrinkToFit(content, &content.FormattedBody, longMessageCutOffHTML, truncateHTML)
+		if encodedContentSize(content) <= maxEncodedContentBytes {
+			return
+		}
+	}
+	shrinkToFit(content, &content.Body, longMessageCutOffText, truncateUTF8)
+}
+
+func shrinkToFit(content *event.MessageEventContent, field *string, marker string, truncate func(string, int) string) {
+	original := *field
+	fits := func(n int) bool {
+		*field = truncate(original, n) + marker
+		return encodedContentSize(content) <= maxEncodedContentBytes
+	}
+	lo, hi := 0, len(original)
+	for lo < hi {
+		mid := (lo + hi + 1) / 2
+		if fits(mid) {
+			lo = mid
+		} else {
+			hi = mid - 1
+		}
+	}
+	*field = truncate(original, lo) + marker
+}
+
+func truncateUTF8(s string, maxBytes int) string {
+	if len(s) <= maxBytes {
+		return s
+	}
+	for maxBytes > 0 && !utf8.RuneStart(s[maxBytes]) {
+		maxBytes--
+	}
+	return s[:maxBytes]
+}
+
+func truncateHTML(s string, maxBytes int) string {
+	s = truncateUTF8(s, maxBytes)
+	if idx := strings.LastIndexByte(s, '<'); idx > strings.LastIndexByte(s, '>') {
+		s = s[:idx]
+	}
+	if idx := strings.LastIndexByte(s, '&'); idx > strings.LastIndexByte(s, ';') {
+		s = s[:idx]
+	}
+	return s
+}