diff --git a/README.md b/README.md
index 3a1b0dad..826f1425 100644
--- a/README.md
+++ b/README.md
@@ -417,6 +417,10 @@ This produces:
 
 Equivalent to [group](#group), but returns nested arrays instead of nested maps.
 
+<a name="flatGroup" href="#flatGroup">#</a> d3.<b>flatGroup</b>(<i>iterable</i>, <i>...keys</i>) · [Source](https://github.com/d3/d3-array/blob/master/src/group.js), [Examples](https://observablehq.com/@d3/d3-flatgroup)
+
+Equivalent to [group](#group), but returns a flat array of [*key0*, *key1*, …, *values*] instead of nested maps.
+
 <a name="index" href="#index">#</a> d3.<b>index</b>(<i>iterable</i>, <i>...keys</i>) · [Source](https://github.com/d3/d3-array/blob/master/src/group.js), [Examples](https://observablehq.com/@d3/d3-group)
 
 Equivalent to [group](#group) but returns a unique value per compound key instead of an array, throwing if the key is not unique.
@@ -507,6 +511,10 @@ To convert a Map to an Array, use [Array.from](https://developer.mozilla.org/en-
 
 Equivalent to [rollup](#rollup), but returns nested arrays instead of nested maps.
 
+<a name="flatRollup" href="#flatRollup">#</a> d3.<b>flatRollup</b>(<i>iterable</i>, <i>...keys</i>) · [Source](https://github.com/d3/d3-array/blob/master/src/group.js), [Examples](https://observablehq.com/@d3/d3-flatgroup)
+
+Equivalent to [rollup](#rollup), but returns a flat array of [*key0*, *key1*, …, *value*] instead of nested maps.
+
 <a name="groupSort" href="#groupSort">#</a> d3.<b>groupSort</b>(<i>iterable</i>, <i>comparator</i>, <i>key</i>) · [Source](https://github.com/d3/d3-array/blob/master/src/groupSort.js), [Examples](https://observablehq.com/@d3/d3-groupsort)
 <br><a name="groupSort" href="#groupSort">#</a> d3.<b>groupSort</b>(<i>iterable</i>, <i>accessor</i>, <i>key</i>)
 
diff --git a/src/group.js b/src/group.js
index 77ae4dc7..0c30d7e9 100644
--- a/src/group.js
+++ b/src/group.js
@@ -9,6 +9,21 @@ export function groups(values, ...keys) {
   return nest(values, Array.from, identity, keys);
 }
 
+function flatten(groups, keys) {
+  for (let i = 1, n = keys.length; i < n; ++i) {
+    groups = groups.flatMap(g => g.pop().map(([key, value]) => [...g, key, value]));
+  }
+  return groups;
+}
+
+export function flatGroup(values, ...keys) {
+  return flatten(groups(values, ...keys), keys);
+}
+
+export function flatRollup(values, reduce, ...keys) {
+  return flatten(rollups(values, reduce, ...keys), keys);
+}
+
 export function rollup(values, reduce, ...keys) {
   return nest(values, identity, reduce, keys);
 }
diff --git a/src/index.js b/src/index.js
index 44c5ac66..209d7b2f 100644
--- a/src/index.js
+++ b/src/index.js
@@ -8,7 +8,7 @@ export {default as descending} from "./descending.js";
 export {default as deviation} from "./deviation.js";
 export {default as extent} from "./extent.js";
 export {Adder, fsum, fcumsum} from "./fsum.js";
-export {default as group, groups, index, indexes, rollup, rollups} from "./group.js";
+export {default as group, flatGroup, flatRollup, groups, index, indexes, rollup, rollups} from "./group.js";
 export {default as groupSort} from "./groupSort.js";
 export {default as bin, default as histogram} from "./bin.js"; // Deprecated; use bin.
 export {default as thresholdFreedmanDiaconis} from "./threshold/freedmanDiaconis.js";
diff --git a/test/flatGroup-test.js b/test/flatGroup-test.js
new file mode 100644
index 00000000..959d3771
--- /dev/null
+++ b/test/flatGroup-test.js
@@ -0,0 +1,21 @@
+import assert from "assert";
+import {flatGroup} from "../src/index.js";
+
+const data = [
+  {name: "jim",   amount: "34.0",   date: "11/12/2015"},
+  {name: "carl",  amount: "120.11", date: "11/12/2015"},
+  {name: "stacy", amount: "12.01",  date: "01/04/2016"},
+  {name: "stacy", amount: "34.05",  date: "01/04/2016"}
+];
+
+it("flatGroup(data, accessor, accessor) returns the expected array", () => {
+  assert.deepStrictEqual(
+    flatGroup(data, d => d.name, d => d.amount),
+    [
+      ['jim', '34.0', [{name: 'jim', amount: '34.0', date: '11/12/2015'}]],
+      ['carl', '120.11', [{name: 'carl', amount: '120.11', date: '11/12/2015'}]],
+      ['stacy', '12.01', [{name: 'stacy', amount: '12.01', date: '01/04/2016'}]],
+      ['stacy', '34.05', [{name: 'stacy', amount: '34.05', date: '01/04/2016'}]]
+    ]
+  );
+});
diff --git a/test/flatRollup-test.js b/test/flatRollup-test.js
new file mode 100644
index 00000000..04b05496
--- /dev/null
+++ b/test/flatRollup-test.js
@@ -0,0 +1,32 @@
+import assert from "assert";
+import {flatRollup} from "../src/index.js";
+
+const data = [
+  {name: "jim",   amount: "34.0",   date: "11/12/2015"},
+  {name: "carl",  amount: "120.11", date: "11/12/2015"},
+  {name: "stacy", amount: "12.01",  date: "01/04/2016"},
+  {name: "stacy", amount: "34.05",  date: "01/04/2016"}
+];
+
+it("flatRollup(data, reduce, accessor) returns the expected array", () => {
+  assert.deepStrictEqual(
+    flatRollup(data, v => v.length, d => d.name),
+    [
+      ['jim', 1],
+      ['carl', 1],
+      ['stacy', 2]
+    ]
+  );
+});
+
+it("flatRollup(data, reduce, accessor, accessor) returns the expected array", () => {
+  assert.deepStrictEqual(
+    flatRollup(data, v => v.length, d => d.name, d => d.amount),
+    [
+      ['jim', '34.0', 1],
+      ['carl', '120.11', 1],
+      ['stacy', '12.01', 1],
+      ['stacy', '34.05', 1]
+    ]
+  );
+});