diff --git a/htslib/bgzf.h b/htslib/bgzf.h
index 8e6b9b17ed..b3318af9ad 100644
--- a/htslib/bgzf.h
+++ b/htslib/bgzf.h
@@ -35,6 +35,13 @@
 
 #include "hts_defs.h"
 
+// Ensure ssize_t exists within this header. All #includes must precede this,
+// and ssize_t must be undefined again at the end of this header.
+#if defined _MSC_VER && defined _INTPTR_T_DEFINED && !defined ssize_t
+#define HTSLIB_SSIZE_T
+#define ssize_t intptr_t
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -450,4 +457,9 @@ typedef struct BGZF BGZF;
 }
 #endif
 
+#ifdef HTSLIB_SSIZE_T
+#undef HTSLIB_SSIZE_T
+#undef ssize_t
+#endif
+
 #endif
diff --git a/htslib/hfile.h b/htslib/hfile.h
index 987acb7c8f..5624ba6d9e 100644
--- a/htslib/hfile.h
+++ b/htslib/hfile.h
@@ -32,6 +32,13 @@ DEALINGS IN THE SOFTWARE.  */
 
 #include "hts_defs.h"
 
+// Ensure ssize_t exists within this header. All #includes must precede this,
+// and ssize_t must be undefined again at the end of this header.
+#if defined _MSC_VER && defined _INTPTR_T_DEFINED && !defined ssize_t
+#define HTSLIB_SSIZE_T
+#define ssize_t intptr_t
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -368,4 +375,9 @@ int hfile_has_plugin(const char *name);
 }
 #endif
 
+#ifdef HTSLIB_SSIZE_T
+#undef HTSLIB_SSIZE_T
+#undef ssize_t
+#endif
+
 #endif
diff --git a/htslib/hts_os.h b/htslib/hts_os.h
index b71cb89e77..66a8d0571e 100644
--- a/htslib/hts_os.h
+++ b/htslib/hts_os.h
@@ -77,4 +77,10 @@ extern int is_cygpty(int fd);
 #define random rand
 #endif
 
+/* MSVC does not provide ssize_t in its <sys/types.h>. This ensures the type
+   is available (unless suppressed by defining HTS_NO_SSIZE_T first). */
+#if defined _MSC_VER && defined _INTPTR_T_DEFINED && !defined HTS_NO_SSIZE_T && !defined ssize_t
+#define ssize_t intptr_t
+#endif
+
 #endif // HTSLIB_HTS_OS_H
diff --git a/htslib/knetfile.h b/htslib/knetfile.h
index da9cdc5e8e..57a0113a31 100644
--- a/htslib/knetfile.h
+++ b/htslib/knetfile.h
@@ -44,6 +44,13 @@
 #define netclose(fd) closesocket(fd)
 #endif
 
+// Ensure ssize_t exists within this header. All #includes must precede this,
+// and ssize_t must be undefined again at the end of this header.
+#if defined _MSC_VER && defined _INTPTR_T_DEFINED && !defined ssize_t
+#define HTSLIB_SSIZE_T
+#define ssize_t intptr_t
+#endif
+
 // FIXME: currently I/O is unbuffered
 
 #define KNF_TYPE_LOCAL 1
@@ -102,4 +109,9 @@ extern "C" {
 }
 #endif
 
+#ifdef HTSLIB_SSIZE_T
+#undef HTSLIB_SSIZE_T
+#undef ssize_t
+#endif
+
 #endif
diff --git a/htslib/kstring.h b/htslib/kstring.h
index 150757ca68..7bc7e414e5 100644
--- a/htslib/kstring.h
+++ b/htslib/kstring.h
@@ -55,6 +55,13 @@
 #endif
 #endif
 
+// Ensure ssize_t exists within this header. All #includes must precede this,
+// and ssize_t must be undefined again at the end of this header.
+#if defined _MSC_VER && defined _INTPTR_T_DEFINED && !defined ssize_t
+#define HTSLIB_SSIZE_T
+#define ssize_t intptr_t
+#endif
+
 /* kstring_t is a simple non-opaque type whose fields are likely to be
  * used directly by user code (but see also ks_str() and ks_len() below).
  * A kstring_t object is initialised by either of
@@ -396,4 +403,9 @@ static inline int *ksplit(kstring_t *s, int delimiter, int *n)
 	return offsets;
 }
 
+#ifdef HTSLIB_SSIZE_T
+#undef HTSLIB_SSIZE_T
+#undef ssize_t
+#endif
+
 #endif
diff --git a/htslib/sam.h b/htslib/sam.h
index 79ac26e4b5..65cd123e2d 100644
--- a/htslib/sam.h
+++ b/htslib/sam.h
@@ -33,6 +33,13 @@ DEALINGS IN THE SOFTWARE.  */
 #include "hts.h"
 #include "hts_endian.h"
 
+// Ensure ssize_t exists within this header. All #includes must precede this,
+// and ssize_t must be undefined again at the end of this header.
+#if defined _MSC_VER && defined _INTPTR_T_DEFINED && !defined ssize_t
+#define HTSLIB_SSIZE_T
+#define ssize_t intptr_t
+#endif
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -2266,4 +2273,9 @@ int bam_mods_at_qpos(const bam1_t *b, int qpos, hts_base_mod_state *state,
 }
 #endif
 
+#ifdef HTSLIB_SSIZE_T
+#undef HTSLIB_SSIZE_T
+#undef ssize_t
+#endif
+
 #endif