diff --git a/src/bin/pg_basebackup/pg_basebackup.c b/src/bin/pg_basebackup/pg_basebackup.c
index 7d17f8b33e..22836ca01a 100644
--- a/src/bin/pg_basebackup/pg_basebackup.c
+++ b/src/bin/pg_basebackup/pg_basebackup.c
@@ -340,12 +340,22 @@ tablespace_list_append(const char *arg)
 		pg_fatal("invalid tablespace mapping format \"%s\", must be \"OLDDIR=NEWDIR\"", arg);
 
 	/*
-	 * This check isn't absolutely necessary.  But all tablespaces are created
-	 * with absolute directories, so specifying a non-absolute path here would
-	 * just never match, possibly confusing users.  It's also good to be
-	 * consistent with the new_dir check.
+	 * All tablespaces are created with absolute directories, so specifying a
+	 * non-absolute path here would just never match, possibly confusing users.
+	 * Since we don't know whether the remote side is Windows or not, and it
+	 * might be different than the local side, permit any path that could be
+	 * absolute under either set of rules.
+	 *
+	 * (There is little practical risk of confusion here, because someone
+	 * running entirely on Linux isn't likely to have a relative path that
+	 * begins with a backslash or something that looks like a drive
+	 * specification. If they do, and they also incorrectly believe that
+	 * a relative path is acceptable here, we'll silently fail to warn them
+	 * of their mistake, and the -T option will just not get applied, same
+	 * as if they'd specified -T for a nonexistent tablespace.)
 	 */
-	if (!is_absolute_path(cell->old_dir))
+	if (!is_nonwindows_absolute_path(cell->old_dir) &&
+		!is_windows_absolute_path(cell->old_dir))
 		pg_fatal("old directory is not an absolute path in tablespace mapping: %s",
 				 cell->old_dir);
 
diff --git a/src/include/port.h b/src/include/port.h
index 69d8818d61..02189d59b1 100644
--- a/src/include/port.h
+++ b/src/include/port.h
@@ -78,28 +78,32 @@ extern void get_parent_directory(char *path);
 extern char **pgfnames(const char *path);
 extern void pgfnames_cleanup(char **filenames);
 
+#define IS_NONWINDOWS_DIR_SEP(ch)	((ch) == '/')
+#define is_nonwindows_absolute_path(filename) \
+( \
+	IS_NONWINDOWS_DIR_SEP((filename)[0]) \
+)
+
+#define IS_WINDOWS_DIR_SEP(ch)	((ch) == '/' || (ch) == '\\')
+/* See path_is_relative_and_below_cwd() for how we handle 'E:abc'. */
+#define is_windows_absolute_path(filename) \
+( \
+	IS_WINDOWS_DIR_SEP((filename)[0]) || \
+	(isalpha((unsigned char) ((filename)[0])) && (filename)[1] == ':' && \
+	 IS_WINDOWS_DIR_SEP((filename)[2])) \
+)
+
 /*
- *	is_absolute_path
+ *	is_absolute_path and IS_DIR_SEP
  *
- *	By making this a macro we avoid needing to include path.c in libpq.
+ *	By using macros here we avoid needing to include path.c in libpq.
  */
 #ifndef WIN32
-#define IS_DIR_SEP(ch)	((ch) == '/')
-
-#define is_absolute_path(filename) \
-( \
-	IS_DIR_SEP((filename)[0]) \
-)
+#define IS_DIR_SEP(ch) IS_NONWINDOWS_DIR_SEP(ch)
+#define is_absolute_path(filename) is_nonwindows_absolute_path(filename)
 #else
-#define IS_DIR_SEP(ch)	((ch) == '/' || (ch) == '\\')
-
-/* See path_is_relative_and_below_cwd() for how we handle 'E:abc'. */
-#define is_absolute_path(filename) \
-( \
-	IS_DIR_SEP((filename)[0]) || \
-	(isalpha((unsigned char) ((filename)[0])) && (filename)[1] == ':' && \
-	 IS_DIR_SEP((filename)[2])) \
-)
+#define IS_DIR_SEP(ch) IS_WINDOWS_DIR_SEP(ch)
+#define is_absolute_path(filename) is_windows_absolute_path(filename)
 #endif
 
 /*