diff --git a/src/datetime.js b/src/datetime.js
index 28cd40ce6..1856b0b3a 100644
--- a/src/datetime.js
+++ b/src/datetime.js
@@ -1502,7 +1502,12 @@ export default class DateTime {
    * @return {string}
    */
   toISODate() {
-    return toTechFormat(this, "yyyy-MM-dd");
+    let format = "yyyy-MM-dd";
+    if (this.year > 9999) {
+      format = "+" + format;
+    }
+
+    return toTechFormat(this, format);
   }
 
   /**
diff --git a/test/datetime/format.test.js b/test/datetime/format.test.js
index c6c14095b..6251d7507 100644
--- a/test/datetime/format.test.js
+++ b/test/datetime/format.test.js
@@ -54,6 +54,15 @@ test("DateTime#toISODate() returns null for invalid DateTimes", () => {
   expect(invalid.toISODate()).toBe(null);
 });
 
+test("DateTime#toISODate() returns ISO 8601 date in format [±YYYYY]", () => {
+  expect(DateTime.fromObject({ year: 118040, month: 5, day: 25, zone: "utc" }).toISODate()).toBe(
+    "+118040-05-25"
+  );
+  expect(DateTime.fromObject({ year: -118040, month: 5, day: 25, zone: "utc" }).toISODate()).toBe(
+    "-118040-05-25"
+  );
+});
+
 //------
 // #toISOWeekDate()
 //------