diff --git a/doc/script_commands.txt b/doc/script_commands.txt
index 75a216fd1c..2af352e805 100644
--- a/doc/script_commands.txt
+++ b/doc/script_commands.txt
@@ -9761,7 +9761,7 @@ Returns true on success and false on failure.
 
 This function will return the specified <type> value for an achievement of the
 attached player or the supplied <char id>. If the player doesn't have the
-achievement active (no progress has been made), if the achievement doesn't
+achievement active (no progress has been made): if the achievement doesn't
 exist -1 will be returned, or -2 will be returned on any other error such as
 an invalid <type>.
 
@@ -9798,4 +9798,16 @@ This function will return if the achievement exists on the player or the supplie
 <char id>.
 Returns true on success and false on failure.
 
----------------------------------------
\ No newline at end of file
+---------------------------------------
+
+*achievementupdate(<achievement id>,<type>,<value>{,<char id>})
+
+This function will update an achievement's value for an achievement of the attached
+player or the supplied <char id>. If the player does not have the achievement active
+(no progress has been made) it will be added to the player's log first before updating
+the <type> value.
+Returns true on success and false on failure.
+
+See 'achievementinfo' for valid <type> values. Excludes ACHIEVEINFO_LEVEL and ACHIEVEINFO_SCORE.
+
+---------------------------------------
diff --git a/src/map/script.c b/src/map/script.c
index e845843bec..4ad865a310 100644
--- a/src/map/script.c
+++ b/src/map/script.c
@@ -23365,6 +23365,58 @@ BUILDIN_FUNC(achievementexists) {
 	return SCRIPT_CMD_SUCCESS;
 }
 
+/**
+ * Updates an achievement's value.
+ * achievementupdate(<achievement ID>,<type>,<value>{,<char ID>});
+ */
+BUILDIN_FUNC(achievementupdate) {
+	struct map_session_data *sd;
+	int i, achievement_id, type, value;
+
+	achievement_id = script_getnum(st, 2);
+	type = script_getnum(st, 3);
+	value = script_getnum(st, 4);
+
+	if (!script_charid2sd(5, sd)) {
+		script_pushint(st, false);
+		return SCRIPT_CMD_FAILURE;
+	}
+
+	if (achievement_search(achievement_id) == &achievement_dummy) {
+		ShowWarning("buildin_achievementupdate: Achievement '%d' doesn't exist.\n", achievement_id);
+		script_pushint(st, false);
+		return SCRIPT_CMD_FAILURE;
+	}
+
+	ARR_FIND(0, sd->achievement_data.count, i, sd->achievement_data.achievements[i].achievement_id == achievement_id);
+	if (i == sd->achievement_data.count)
+		achievement_add(sd, achievement_id);
+
+	ARR_FIND(0, sd->achievement_data.count, i, sd->achievement_data.achievements[i].achievement_id == achievement_id);
+	if (i == sd->achievement_data.count) {
+		script_pushint(st, false);
+		return SCRIPT_CMD_SUCCESS;
+	}
+
+	if (type >= ACHIEVEINFO_COUNT1 && type <= ACHIEVEINFO_COUNT10)
+		sd->achievement_data.achievements[i].count[type - 1] = value;
+	else if (type == ACHIEVEINFO_COMPLETE)
+		sd->achievement_data.achievements[i].complete = value ?  true : false;
+	else if (type == ACHIEVEINFO_COMPLETEDATE)
+		sd->achievement_data.achievements[i].completeDate = value;
+	else if (type == ACHIEVEINFO_GOTREWARD)
+		sd->achievement_data.achievements[i].gotReward = value ? true : false;
+	else {
+		ShowWarning("buildin_achievementupdate: Unknown type '%d'.\n", type);
+		script_pushint(st, false);
+		return SCRIPT_CMD_FAILURE;
+	}
+
+	achievement_update_achievement(sd, achievement_id, false);
+	script_pushint(st, true);
+	return SCRIPT_CMD_SUCCESS;
+}
+
 #include "../custom/script.inc"
 
 // declarations that were supposed to be exported from npc_chat.c
@@ -24002,6 +24054,7 @@ struct script_function buildin_func[] = {
 	BUILDIN_DEF(achievementremove,"i?"),
 	BUILDIN_DEF(achievementcomplete,"i?"),
 	BUILDIN_DEF(achievementexists,"i?"),
+	BUILDIN_DEF(achievementupdate,"iii?"),
 
 #include "../custom/script_def.inc"