diff --git a/Iterator.concat/auto.js b/Iterator.concat/auto.js
new file mode 100644
index 0000000..8ebf606
--- /dev/null
+++ b/Iterator.concat/auto.js
@@ -0,0 +1,3 @@
+'use strict';
+
+require('./shim')();
diff --git a/Iterator.concat/implementation.js b/Iterator.concat/implementation.js
new file mode 100644
index 0000000..339dcec
--- /dev/null
+++ b/Iterator.concat/implementation.js
@@ -0,0 +1,112 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var AdvanceStringIndex = require('es-abstract/2024/AdvanceStringIndex');
+var Call = require('es-abstract/2024/Call');
+var CompletionRecord = require('es-abstract/2024/CompletionRecord');
+var CreateIteratorFromClosure = require('../aos/CreateIteratorFromClosure');
+var GetIteratorDirect = require('../aos/GetIteratorDirect');
+var GetMethod = require('es-abstract/2024/GetMethod');
+var IsArray = require('es-abstract/2024/IsArray');
+var IteratorClose = require('es-abstract/2024/IteratorClose');
+var IteratorStepValue = require('es-abstract/2024/IteratorStepValue');
+var ThrowCompletion = require('es-abstract/2024/ThrowCompletion');
+var Type = require('es-abstract/2024/Type');
+
+var forEach = require('es-abstract/helpers/forEach');
+var getIteratorMethod = require('es-abstract/helpers/getIteratorMethod');
+
+var iterHelperProto = require('../IteratorHelperPrototype');
+
+var SLOT = require('internal-slot');
+
+module.exports = function concat() {
+	if (this instanceof concat) {
+		throw new $TypeError('`Iterator.concat` is not a constructor');
+	}
+
+	var iterables = []; // step 1
+	var index = 0;
+
+	forEach(arguments, function (item) { // step 2
+		if (Type(item) !== 'Object') {
+			throw new $TypeError('`Iterator.concat` requires all arguments to be objects'); // step 2.1
+		}
+		// var method = GetMethod(item, Symbol.iterator); // step 2.2
+		var method = getIteratorMethod(
+			{
+				AdvanceStringIndex: AdvanceStringIndex,
+				GetMethod: GetMethod,
+				IsArray: IsArray
+			},
+			item
+		);
+		if (typeof method === 'undefined') {
+			throw new $TypeError('`Iterator.concat` requires all arguments to be iterable'); // step 2.3
+		}
+		iterables[iterables.length] = { '[[OpenMethod]]': method, '[[Iterable]]': item }; // step 2.4
+	});
+
+	var sentinel = {};
+	var innerIterator = sentinel;
+	var closeIfAbrupt = function (abruptCompletion) {
+		if (!(abruptCompletion instanceof CompletionRecord)) {
+			throw new $TypeError('`abruptCompletion` must be a Completion Record');
+		}
+		if (innerIterator !== sentinel) {
+			IteratorClose(
+				innerIterator,
+				abruptCompletion
+			);
+		}
+	};
+
+	var closure = function () { // step 3
+		if (index < iterables.length) {
+		// forEach(iterables, function (iterable) { // step 3.a
+			var iteratorRecord;
+			if (innerIterator === sentinel) {
+				var iterable = iterables[index];
+				var iter = Call(iterable['[[OpenMethod]]'], iterable['[[Iterable]]']); // step 3.a.i
+				if (Type(iter) !== 'Object') {
+					closeIfAbrupt(ThrowCompletion(new $TypeError('???'))); // step 3.a.ii
+				}
+				iteratorRecord = GetIteratorDirect(iter); // step 3.a.iii
+				innerIterator = iteratorRecord;
+			} else {
+				iteratorRecord = innerIterator;
+			}
+
+			// var innerAlive = true; // step 3.a.iv
+			// while (innerAlive) { // step 3.a.v
+			if (innerIterator !== sentinel) {
+				// step 3.a.v.3.a
+				var innerValue;
+				try {
+					innerValue = IteratorStepValue(iteratorRecord); // step 5.b.ix.4.a
+				} catch (e) {
+					// innerAlive = false;
+					innerIterator = sentinel;
+					index += 1;
+					closeIfAbrupt(ThrowCompletion(e)); // step 3.a.v.3.b
+				}
+				if (iteratorRecord['[[Done]]']) {
+					// innerAlive = false;
+					innerIterator = sentinel;
+					index += 1;
+					return closure();
+				}
+				return innerValue; // // step 3.a.v.3.a
+			}
+		// });
+		}
+
+		// return ReturnCompletion(undefined); // step 3.b
+		return sentinel;
+	};
+	SLOT.set(closure, '[[Sentinel]]', sentinel); // for the userland implementation
+	SLOT.set(closure, '[[CloseIfAbrupt]]', closeIfAbrupt); // for the userland implementation
+
+	return CreateIteratorFromClosure(closure, 'Iterator Helper', iterHelperProto, []); // step 4
+};
diff --git a/Iterator.concat/index.js b/Iterator.concat/index.js
new file mode 100644
index 0000000..4c50b07
--- /dev/null
+++ b/Iterator.concat/index.js
@@ -0,0 +1,18 @@
+'use strict';
+
+var callBind = require('call-bind');
+var define = require('define-properties');
+
+var implementation = require('./implementation');
+var getPolyfill = require('./polyfill');
+var shim = require('./shim');
+
+var bound = callBind(getPolyfill(), null);
+
+define(bound, {
+	getPolyfill: getPolyfill,
+	implementation: implementation,
+	shim: shim
+});
+
+module.exports = bound;
diff --git a/Iterator.concat/polyfill.js b/Iterator.concat/polyfill.js
new file mode 100644
index 0000000..25bbab6
--- /dev/null
+++ b/Iterator.concat/polyfill.js
@@ -0,0 +1,9 @@
+'use strict';
+
+var implementation = require('./implementation');
+
+var $Iterator = require('../Iterator');
+
+module.exports = function getPolyfill() {
+	return typeof $Iterator.concat === 'function' ? $Iterator.concat : implementation;
+};
diff --git a/Iterator.concat/shim.js b/Iterator.concat/shim.js
new file mode 100644
index 0000000..a45f0d8
--- /dev/null
+++ b/Iterator.concat/shim.js
@@ -0,0 +1,18 @@
+'use strict';
+
+var getPolyfill = require('./polyfill');
+var define = require('define-properties');
+
+var getIteratorPolyfill = require('../Iterator/polyfill');
+
+module.exports = function shimIteratorConcat() {
+	var $Iterator = getIteratorPolyfill();
+	var polyfill = getPolyfill();
+	define(
+		$Iterator,
+		{ concat: polyfill },
+		{ concat: function () { return $Iterator.concat !== polyfill; } }
+	);
+
+	return polyfill;
+};
diff --git a/README.md b/README.md
index 1bb73d7..b26a724 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@
 
 An ESnext spec-compliant sync iterator helpers shim/polyfill/replacement that works as far down as ES3.
 
-This package implements the [es-shim API](https://github.com/es-shims/api) “multi” interface. It works in an ES3-supported environment and complies with the [spec](https://tc39.es/proposal-iterator-helpers/).
+This package implements the [es-shim API](https://github.com/es-shims/api) “multi” interface. It works in an ES3-supported environment and complies with the [iterator helpers spec](https://tc39.es/proposal-iterator-helpers/) and the [iterator sequencing spec](https://tc39.es/proposal-iterator-sequencing/).
 
 Because the `Iterator.prototype` methods depend on a receiver (the `this` value), the main export in each subdirectory takes the iterator to operate on as the first argument.
 
@@ -19,6 +19,7 @@ The main export of the package itself is simply an array of the available direct
 
  - [`Iterator` constructor](https://tc39.es/proposal-iterator-helpers/#sec-iterator-constructor)
  - [`Iterator.prototype`](https://tc39.es/proposal-iterator-helpers/#sec-iterator.prototype)
+ - [`Iterator.concat`](https://tc39.es/proposal-iterator-sequencing/)
  - [`Iterator.from`](https://tc39.es/proposal-iterator-helpers/#sec-iterator.from)
  - [`Iterator.prototype.constructor`](https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.constructor)
  - [`Iterator.prototype.drop`](https://tc39.es/proposal-iterator-helpers/#sec-iteratorprototype.drop)
@@ -37,6 +38,7 @@ The main export of the package itself is simply an array of the available direct
 
  - node v22, Chrome >= v122: has a [bug](https://issues.chromium.org/issues/336839115)
  - node < v22, Chrome < v122, Safari <= v17.1, Firefox <= v125: not implemented
+ - all environments lack Iterator.concat
 
 ## Getting started
 
diff --git a/index.json b/index.json
index 0e3ce66..9ac26be 100644
--- a/index.json
+++ b/index.json
@@ -1,5 +1,6 @@
 [
 	"Iterator",
+	"Iterator.concat",
 	"Iterator.from",
 	"Iterator.prototype",
 	"Iterator.prototype.constructor",
diff --git a/package.json b/package.json
index 2ae89a6..f4e3659 100644
--- a/package.json
+++ b/package.json
@@ -17,6 +17,11 @@
 		"./Iterator.prototype/polyfill": "./Iterator.prototype/polyfill.js",
 		"./Iterator.prototype/implementation": "./Iterator.prototype/implementation.js",
 		"./Iterator.prototype/shim": "./Iterator.prototype/shim.js",
+		"./Iterator.concat": "./Iterator.concat/index.js",
+		"./Iterator.concat/auto": "./Iterator.concat/auto.js",
+		"./Iterator.concat/polyfill": "./Iterator.concat/polyfill.js",
+		"./Iterator.concat/implementation": "./Iterator.concat/implementation.js",
+		"./Iterator.concat/shim": "./Iterator.concat/shim.js",
 		"./Iterator.from": "./Iterator.from/index.js",
 		"./Iterator.from/auto": "./Iterator.from/auto.js",
 		"./Iterator.from/polyfill": "./Iterator.from/polyfill.js",
diff --git a/shim.js b/shim.js
index 2ba8c91..077f751 100644
--- a/shim.js
+++ b/shim.js
@@ -2,6 +2,7 @@
 
 var shimIterator = require('./Iterator/shim');
 var shimIteratorFrom = require('./Iterator.from/shim');
+var shimIteratorConcat = require('./Iterator.concat/shim');
 var shimIteratorProto = require('./Iterator.prototype/shim');
 var shimIteratorCtor = require('./Iterator.prototype.constructor/shim');
 var shimIteratorDrop = require('./Iterator.prototype.drop/shim');
@@ -19,6 +20,7 @@ var shimIteratorToArray = require('./Iterator.prototype.toArray/shim');
 module.exports = function shimIteratorHelpers() {
 	shimIterator();
 	shimIteratorFrom();
+	shimIteratorConcat();
 	shimIteratorProto();
 	shimIteratorCtor();
 	shimIteratorDrop();
diff --git a/test/Iterator.concat.js b/test/Iterator.concat.js
new file mode 100644
index 0000000..a244c2b
--- /dev/null
+++ b/test/Iterator.concat.js
@@ -0,0 +1,129 @@
+'use strict';
+
+var defineProperties = require('define-properties');
+var test = require('tape');
+var callBind = require('call-bind');
+var functionsHaveNames = require('functions-have-names')();
+var forEach = require('for-each');
+var debug = require('object-inspect');
+var v = require('es-value-fixtures');
+var hasSymbols = require('has-symbols/shams')();
+var mockProperty = require('mock-property');
+
+var index = require('../Iterator.concat');
+var impl = require('../Iterator.concat/implementation');
+var from = require('../Iterator.from/polyfill')();
+
+var isEnumerable = Object.prototype.propertyIsEnumerable;
+
+var testIterator = require('./helpers/testIterator');
+
+module.exports = {
+	tests: function (concat, name, t) {
+		t['throws'](
+			function () { return new concat(); }, // eslint-disable-line new-cap
+			TypeError,
+			'`' + name + '` itself is not a constructor'
+		);
+		t['throws'](
+			function () { return new concat({}); }, // eslint-disable-line new-cap
+			TypeError,
+			'`' + name + '` itself is not a constructor, with an argument'
+		);
+
+		forEach(v.primitives.concat(v.objects), function (nonIterator) {
+			if (typeof nonIterator !== 'string') {
+				t['throws'](
+					function () { concat(nonIterator).next(); },
+					TypeError,
+					debug(nonIterator) + ' is not an iterable Object'
+				);
+			}
+		});
+
+		t.test('actual iteration', { skip: !hasSymbols }, function (st) {
+			forEach(v.nonFunctions, function (nonFunction) {
+				var badIterable = {};
+				badIterable[Symbol.iterator] = nonFunction;
+				st['throws'](
+					function () { concat([], badIterable, []).next(); },
+					TypeError,
+					debug(badIterable) + ' is not a function'
+				);
+			});
+
+			forEach(v.strings, function (string) {
+				st['throws'](
+					function () { concat(string); },
+					TypeError,
+					'non-objects are not considered iterable'
+				);
+				var stringIt = concat(['a'], [string], ['c']);
+				testIterator(stringIt, ['a', string, 'c'], st, 'string iterator: ' + debug(string));
+			});
+
+			var arrayIt = concat([1, 2, 3]);
+			st.equal(typeof arrayIt.next, 'function', 'has a `next` function');
+
+			st.test('real iterators', { skip: !hasSymbols }, function (s2t) {
+				var iter = [1, 2][Symbol.iterator]();
+				testIterator(concat(iter, [3]), [1, 2, 3], s2t, 'array iterator + array yields combined results');
+
+				s2t.end();
+			});
+
+			st.test('observability in a replaced String iterator', function (s2t) {
+				var originalStringIterator = String.prototype[Symbol.iterator];
+				var observedType;
+				s2t.teardown(mockProperty(String.prototype, Symbol.iterator, {
+					get: function () {
+						'use strict'; // eslint-disable-line strict, lines-around-directive
+
+						observedType = typeof this;
+						return originalStringIterator;
+					}
+				}));
+
+				concat(from(''));
+				s2t.equal(observedType, 'string', 'string primitive -> primitive receiver in Symbol.iterator getter');
+				concat(from(Object('')));
+				s2t.equal(observedType, 'object', 'boxed string -> boxed string in Symbol.iterator getter');
+
+				s2t.end();
+			});
+
+			st.end();
+		});
+	},
+	index: function () {
+		test('Iterator.concat: index', function (t) {
+			module.exports.tests(index, 'Iterator.concat', t);
+
+			t.end();
+		});
+	},
+	implementation: function () {
+		test('Iterator.concat: implementation', function (t) {
+			module.exports.tests(impl, 'Iterator.concat', t);
+
+			t.end();
+		});
+	},
+	shimmed: function () {
+		test('Iterator.concat: shimmed', function (t) {
+			t.test('Function name', { skip: !functionsHaveNames }, function (st) {
+				st.equal(Iterator.concat.name, 'concat', 'Iterator.concat has name "concat"');
+				st.end();
+			});
+
+			t.test('enumerability', { skip: !defineProperties.supportsDescriptors }, function (et) {
+				et.equal(false, isEnumerable.call(Iterator, 'concat'), 'Iterator.concat is not enumerable');
+				et.end();
+			});
+
+			module.exports.tests(callBind(Iterator.concat, Iterator), 'Iterator.concat', t);
+
+			t.end();
+		});
+	}
+};