diff --git a/icu4c/source/common/lstmbe.cpp b/icu4c/source/common/lstmbe.cpp
index 3793abceb3f..70fd038b2c0 100644
--- a/icu4c/source/common/lstmbe.cpp
+++ b/icu4c/source/common/lstmbe.cpp
@@ -1,8 +1,8 @@
 // © 2021 and later: Unicode, Inc. and others.
 // License & terms of use: http://www.unicode.org/copyright.html
 
+#include <complex>
 #include <utility>
-#include <ctgmath>
 
 #include "unicode/utypes.h"