diff --git a/icu4c/source/common/ucnv_io.h b/icu4c/source/common/ucnv_io.h
index 3f843c2f82c..deaec172659 100644
--- a/icu4c/source/common/ucnv_io.h
+++ b/icu4c/source/common/ucnv_io.h
@@ -14,6 +14,7 @@
 #define UCNV_IO_H
 
 #include "unicode/utypes.h"
+#include "udataswp.h"
 
 #define UCNV_AMBIGUOUS_ALIAS_MAP_BIT 0x8000
 #define UCNV_CONVERTER_INDEX_MASK 0xFFF
@@ -149,10 +150,6 @@ ucnv_io_getDefaultConverterName(void);
 U_CFUNC void
 ucnv_io_setDefaultConverterName(const char *name);
 
-/* forward declaration */
-struct UDataSwapper;
-typedef struct UDataSwapper UDataSwapper;
-
 /**
  * Swap an ICU converter alias table. See ucnv_io.c.
  * @internal
diff --git a/icu4c/source/common/uprops.h b/icu4c/source/common/uprops.h
index 23470f6c586..06e642ea9c4 100644
--- a/icu4c/source/common/uprops.h
+++ b/icu4c/source/common/uprops.h
@@ -22,6 +22,7 @@
 
 #include "unicode/utypes.h"
 #include "unicode/uset.h"
+#include "udataswp.h"
 
 /* indexes[] entries */
 enum {
@@ -354,10 +355,6 @@ uchar_addPropertyStarts(USet *set, UErrorCode *pErrorCode);
 U_CAPI void U_EXPORT2
 uprv_getInclusions(USet* set, UErrorCode *pErrorCode);
 
-/* forward declaration */
-struct UDataSwapper;
-typedef struct UDataSwapper UDataSwapper;
-
 /**
  * Swap the ICU Unicode properties file. See uchar.c.
  * @internal
diff --git a/icu4c/source/common/utrie.h b/icu4c/source/common/utrie.h
index 5ad99e08cd7..1db7632fcac 100644
--- a/icu4c/source/common/utrie.h
+++ b/icu4c/source/common/utrie.h
@@ -18,6 +18,7 @@
 #define __UTRIE_H__
 
 #include "unicode/utypes.h"
+#include "udataswp.h"
 
 U_CDECL_BEGIN
 
@@ -663,10 +664,6 @@ utrie_serialize(UNewTrie *trie, void *data, int32_t capacity,
                 UBool reduceTo16Bits,
                 UErrorCode *pErrorCode);
 
-/* forward declaration */
-struct UDataSwapper;
-typedef struct UDataSwapper UDataSwapper;
-
 /**
  * Swap a serialized UTrie.
  * @internal