diff --git a/icu4c/source/data/brkitr/line_th.txt b/icu4c/source/data/brkitr/line_th.txt
index 168ed268af..c64d503756 100644
--- a/icu4c/source/data/brkitr/line_th.txt
+++ b/icu4c/source/data/brkitr/line_th.txt
@@ -376,8 +376,12 @@ $Openings $GluedWord  $Closings $paiyannoi   /
 #
 #  Reverse Rules.
 #
-#     Back up to a hard break.
-#     TODO:  make smarter reverse rules for better efficiency
+#     Back up to a hard break or a space that will cause a boundary.
+#     Not all spaces cause line breaks.  $SpaceGlue represents a sequence
+#     containing a space that may inhibit a break from occuring.
 #
-! . . [^$BK | $CR | $LF]*   (. | $LF $CR);
-! .*;
+$SpaceGlue  = ([$ZW $CL $IS $NS $OP]  ($Extend* $SP)) | (($Extend* $SP)+ $OP);
+$ClumpingChars = [^$SP $BK $CR $LF];
+
+!. . $ClumpingChars*  ($SpaceGlue $ClumpingChars*)* (. | $LF $CR);
+