diff --git a/include/tig/string.h b/include/tig/string.h
index d12e22c3b..2ac1a95e6 100644
--- a/include/tig/string.h
+++ b/include/tig/string.h
@@ -15,12 +15,21 @@
 #define TIG_STRING_H
 
 #include "tig/tig.h"
-#include "tig/string.h"
 
 /*
  * Strings.
  */
 
+#if defined(__APPLE__)
+/* See COMPATIBILITY in isspace(3). */
+#define isalnum(c) iswalnum(btowc(c))
+#define iscntrl(c) iswcntrl(btowc(c))
+#define isdigit(c) iswdigit(btowc(c))
+#define ispunct(c) iswpunct(btowc(c))
+#define isspace(c) iswspace(btowc(c))
+#define isxdigit(c) iswxdigit(btowc(c))
+#endif
+
 #define prefixcmp(str1, str2) \
 	strncmp(str1, str2, STRING_SIZE(str2))
 
diff --git a/include/tig/tig.h b/include/tig/tig.h
index 2b0bc53d5..278726ef7 100644
--- a/include/tig/tig.h
+++ b/include/tig/tig.h
@@ -67,6 +67,10 @@
 #include <libgen.h>
 #include <termios.h>
 
+#if defined(__APPLE__)
+#include <wchar.h>
+#endif
+
 #if defined HAVE_PCRE2
 #include <pcre2posix.h>
 #elif defined HAVE_PCRE
diff --git a/src/argv.c b/src/argv.c
index 032b0705b..80f4d6f4b 100644
--- a/src/argv.c
+++ b/src/argv.c
@@ -129,7 +129,7 @@ parse_arg(char **cmd, bool remove_quotes)
 				break;
 		}
 
-		if (!quote && isspace(c))
+		if (!quote && isspace((unsigned char)c))
 			break;
 
 		*next++ = *pos;
@@ -315,7 +315,7 @@ format_expand_arg(struct format_context *format, const char *name, const char *e
 		if (end && msglen > 0 && string_format(msgbuf, "%.*s", msglen, msgstart)) {
 			const char *msg = msgbuf;
 
-			while (isspace(*msg))
+			while (isspace((unsigned char)*msg))
 				msg++;
 			if (*msg)
 				prompt = msg;
diff --git a/src/main.c b/src/main.c
index d9b5f61d7..3ac88c6fc 100644
--- a/src/main.c
+++ b/src/main.c
@@ -444,7 +444,7 @@ main_read(struct view *view, struct buffer *buf, bool force_stop)
 		state->in_header = true;
 		line += STRING_SIZE("commit ");
 		is_boundary = *line == '-';
-		while (*line && !isalnum(*line))
+		while (*line && !isalnum((unsigned char)*line))
 			line++;
 
 		main_flush_commit(view, commit);
@@ -524,7 +524,7 @@ main_read(struct view *view, struct buffer *buf, bool force_stop)
 		line += 4;
 		/* Well, if the title starts with a whitespace character,
 		 * try to be forgiving.  Otherwise we end up with no title. */
-		while (isspace(*line))
+		while (isspace((unsigned char)*line))
 			line++;
 		if (*line == '\0')
 			break;
diff --git a/src/options.c b/src/options.c
index 4e0d26f49..56cbe5025 100644
--- a/src/options.c
+++ b/src/options.c
@@ -319,7 +319,7 @@ parse_step(double *opt, const char *arg)
 {
 	int value = atoi(arg);
 
-	if (!value && !isdigit(*arg))
+	if (!value && !isdigit((unsigned char)*arg))
 		return error("Invalid double or percentage");
 
 	*opt = value;
diff --git a/src/pager.c b/src/pager.c
index 0f81e110d..e15530f7c 100644
--- a/src/pager.c
+++ b/src/pager.c
@@ -126,7 +126,7 @@ pager_common_read(struct view *view, const char *data, enum line_type type, stru
 
 	if (line->type == LINE_COMMIT && view_has_flags(view, VIEW_ADD_PAGER_REFS)) {
 		data += STRING_SIZE("commit ");
-		while (*data && !isalnum(*data))
+		while (*data && !isalnum((unsigned char)*data))
 			data++;
 		add_pager_refs(view, data);
 	}
diff --git a/src/parse.c b/src/parse.c
index 1d91cda0d..983afdb82 100644
--- a/src/parse.c
+++ b/src/parse.c
@@ -23,7 +23,7 @@ parse_size(const char *text)
 	while (*text == ' ')
 		text++;
 
-	while (isdigit(*text))
+	while (isdigit((unsigned char)*text))
 		size = (size * 10) + (*text++ - '0');
 
 	return size;
@@ -98,7 +98,7 @@ parse_number(const char **posref, size_t *number)
 
 	*posref = NULL;
 	pos = strchr(pos + 1, ' ');
-	if (!pos || !isdigit(pos[1]))
+	if (!pos || !isdigit((unsigned char)pos[1]))
 		return false;
 	*number = atoi(pos + 1);
 
@@ -197,7 +197,7 @@ parse_ulong(const char **pos_ptr, unsigned long *value, char skip, bool optional
 	if (end == start)
 		return false;
 
-	while (isspace(*end))
+	while (isspace((unsigned char)*end))
 		end++;
 	*pos_ptr = end;
 	return true;
diff --git a/src/string.c b/src/string.c
index 6146d8aee..a90fc1bb0 100644
--- a/src/string.c
+++ b/src/string.c
@@ -25,7 +25,7 @@ string_isnumber(const char *str)
 	int pos;
 
 	for (pos = 0; str[pos]; pos++) {
-		if (!isdigit(str[pos]))
+		if (!isdigit((unsigned char)str[pos]))
 			return false;
 	}
 
@@ -38,7 +38,7 @@ iscommit(const char *str)
 	int pos;
 
 	for (pos = 0; str[pos]; pos++) {
-		if (!isxdigit(str[pos]))
+		if (!isxdigit((unsigned char)str[pos]))
 			return false;
 	}
 
@@ -73,7 +73,7 @@ string_copy_rev(char *dst, const char *src)
 		return;
 
 	for (srclen = 0; srclen < SIZEOF_REV; srclen++)
-		if (!src[srclen] || isspace(src[srclen]))
+		if (!src[srclen] || isspace((unsigned char)src[srclen]))
 			break;
 
 	string_ncopy_do(dst, SIZEOF_REV, src, srclen);
@@ -83,7 +83,7 @@ void
 string_copy_rev_from_commit_line(char *dst, const char *src)
 {
 	src += STRING_SIZE("commit ");
-	while (*src && !isalnum(*src))
+	while (*src && !isalnum((unsigned char)*src))
 		src++;
 	string_copy_rev(dst, src);
 }
@@ -103,7 +103,7 @@ string_expand(char *dst, size_t dstlen, const char *src, int srclen, int tabsize
 				expanded = dstlen - size - 1;
 			memcpy(dst + size, "        ", expanded);
 			size += expanded;
-		} else if (isspace(c) || iscntrl(c)) {
+		} else if (isspace((unsigned char)c) || iscntrl((unsigned char)c)) {
 			dst[size++] = ' ';
 		} else {
 			dst[size++] = src[pos];
@@ -119,7 +119,7 @@ string_trim_end(char *name)
 {
 	int namelen = strlen(name) - 1;
 
-	while (namelen > 0 && isspace(name[namelen]))
+	while (namelen > 0 && isspace((unsigned char)name[namelen]))
 		name[namelen--] = 0;
 
 	return name;
@@ -128,7 +128,7 @@ string_trim_end(char *name)
 char *
 string_trim(char *name)
 {
-	while (isspace(*name))
+	while (isspace((unsigned char)*name))
 		name++;
 
 	return string_trim_end(name);
@@ -166,7 +166,7 @@ strcmp_numeric(const char *s1, const char *s2)
 	for (; *s1 && *s2 && *s1 == *s2; s1++, s2++) {
 		int c = *s1;
 
-		if (isdigit(c)) {
+		if (isdigit((unsigned char)c)) {
 			number = 10 * number + (c - '0');
 		} else {
 			number = 0;