diff --git a/src/impl/regexParser.js b/src/impl/regexParser.js
index ab095864d..6710d64ea 100644
--- a/src/impl/regexParser.js
+++ b/src/impl/regexParser.js
@@ -67,11 +67,11 @@ function simpleParse(...keys) {
 }
 
 // ISO and SQL parsing
-const offsetRegex = /(?:(Z)|([+-]\d\d)(?::?(\d\d))?)/;
+const offsetRegex = /(?:([Zz])|([+-]\d\d)(?::?(\d\d))?)/;
 const isoExtendedZone = `(?:${offsetRegex.source}?(?:\\[(${ianaRegex.source})\\])?)?`;
 const isoTimeBaseRegex = /(\d\d)(?::?(\d\d)(?::?(\d\d)(?:[.,](\d{1,30}))?)?)?/;
 const isoTimeRegex = RegExp(`${isoTimeBaseRegex.source}${isoExtendedZone}`);
-const isoTimeExtensionRegex = RegExp(`(?:T${isoTimeRegex.source})?`);
+const isoTimeExtensionRegex = RegExp(`(?:[Tt]${isoTimeRegex.source})?`);
 const isoYmdRegex = /([+-]\d{6}|\d{4})(?:-?(\d\d)(?:-?(\d\d))?)?/;
 const isoWeekRegex = /(\d{4})-?W(\d\d)(?:-?(\d))?/;
 const isoOrdinalRegex = /(\d{4})-?(\d{3})/;
diff --git a/test/datetime/regexParse.test.js b/test/datetime/regexParse.test.js
index db84b968f..50d36b709 100644
--- a/test/datetime/regexParse.test.js
+++ b/test/datetime/regexParse.test.js
@@ -114,6 +114,18 @@ test("DateTime.fromISO() can optionally specify a zone", () => {
 
 const isSame = (s, expected) => expect(DateTime.fromISO(s).toObject()).toEqual(expected);
 
+test("DateTime.fromISO() accepts both capital T and lowercase t", () => {
+  // #1610
+  let dt = DateTime.fromISO("2016-05-25T09:08:34.123+06:00");
+  isSame("2016-05-25t09:08:34.123+06:00", dt.toObject());
+});
+
+test("DateTime.fromISO() accepts both capital Z and lowercase z", () => {
+  // #1610
+  let dt = DateTime.fromISO("2016-05-25T09:08:34.123Z");
+  isSame("2016-05-25T09:08:34.123z", dt.toObject());
+});
+
 test("DateTime.fromISO() accepts just the year", () => {
   isSame("2016", {
     year: 2016,