diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
index ee39c5a895f2..693ae29a8668 100644
--- a/config/interpolate_funcs.go
+++ b/config/interpolate_funcs.go
@@ -82,8 +82,56 @@ func interpolationFuncFormat() ast.Function {
 		VariadicType: ast.TypeAny,
 		ReturnType:   ast.TypeString,
 		Callback: func(args []interface{}) (interface{}, error) {
+			// Make a copy of the variadic part of args
+			// to avoid modifying the original.
+			varargs := make([]interface{}, len(args)-1)
+			copy(varargs, args[1:])
+
+			// Convert arguments that are lists into slices.
+			// Confirm along the way that all lists have the same length (n).
+			var n int
+			for i := 1; i < len(args); i++ {
+				s, ok := args[i].(string)
+				if !ok {
+					continue
+				}
+				parts := strings.Split(s, InterpSplitDelim)
+				if len(parts) == 1 {
+					continue
+				}
+				varargs[i-1] = parts
+				if n == 0 {
+					// first list we've seen
+					n = len(parts)
+					continue
+				}
+				if n != len(parts) {
+					return nil, fmt.Errorf("format: mismatched list lengths: %d != %d", n, len(parts))
+				}
+			}
+
+			// Do the formatting.
 			format := args[0].(string)
-			return fmt.Sprintf(format, args[1:]...), nil
+			if n == 0 {
+				// Easy case: No lists.
+				return fmt.Sprintf(format, varargs...), nil
+			}
+
+			// Generate a list of formatted strings.
+			list := make([]string, n)
+			fmtargs := make([]interface{}, len(varargs))
+			for i := 0; i < n; i++ {
+				for j, arg := range varargs {
+					switch arg := arg.(type) {
+					default:
+						fmtargs[j] = arg
+					case []string:
+						fmtargs[j] = arg[i]
+					}
+				}
+				list[i] = fmt.Sprintf(format, fmtargs...)
+			}
+			return strings.Join(list, InterpSplitDelim), nil
 		},
 	}
 }
diff --git a/config/interpolate_funcs_test.go b/config/interpolate_funcs_test.go
index 044d4f843aa4..bd3d7070cd3b 100644
--- a/config/interpolate_funcs_test.go
+++ b/config/interpolate_funcs_test.go
@@ -102,6 +102,41 @@ func TestInterpolateFuncFormat(t *testing.T) {
 				"hello 12345",
 				false,
 			},
+			// Format applies to each list element in turn
+			{
+				`${format("<%s>", split(",", "A,B"))}`,
+				"<A>" + InterpSplitDelim + "<B>",
+				false,
+			},
+			// Format repeats scalar elements
+			{
+				`${join(", ", format("%s=%s", "x", split(",", "A,B,C")))}`,
+				"x=A, x=B, x=C",
+				false,
+			},
+			// Multiple lists are walked in parallel
+			{
+				`${join(", ", format("%s=%s", split(",", "A,B,C"), split(",", "1,2,3")))}`,
+				"A=1, B=2, C=3",
+				false,
+			},
+			// Lists of length zero/one are repeated, just as scalars are
+			{
+				`${join(", ", format("%s=%s", split(",", ""), split(",", "1,2,3")))}`,
+				"=1, =2, =3",
+				false,
+			},
+			{
+				`${join(", ", format("%s=%s", split(",", "A"), split(",", "1,2,3")))}`,
+				"A=1, A=2, A=3",
+				false,
+			},
+			// Mismatched list lengths generate an error
+			{
+				`${format("%s=%2s", split(",", "A,B,C,D"), split(",", "1,2,3"))}`,
+				nil,
+				true,
+			},
 		},
 	})
 }
diff --git a/website/source/docs/configuration/interpolation.html.md b/website/source/docs/configuration/interpolation.html.md
index b24c02ce8795..9d168dd2d76e 100644
--- a/website/source/docs/configuration/interpolation.html.md
+++ b/website/source/docs/configuration/interpolation.html.md
@@ -88,6 +88,13 @@ The supported built-in functions are:
       Good documentation for the syntax can be [found here](http://golang.org/pkg/fmt/).
       Example to zero-prefix a count, used commonly for naming servers:
       `format("web-%03d", count.index+1)`.
+      If one of args is a list, then the format is applied to each element of the list and format returns a list. Non-list arguments are repeated for each list element.
+      For example, to convert a list of DNS addresses to a list of URLs, you might use:
+      `format("https://%s:%s/", aws_instance.foo.*.public_dns, var.port)`.
+      If multiple args are lists, and they have the same number of elements, then the format is applied to the elements of the lists in parallel.
+      Example:
+      `format("instance %v has private ip %v", aws_instance.foo.*.id, aws_instance.foo.*.private_ip)`.
+      Passing lists with different lengths to format results in an error.
 
   * `join(delim, list)` - Joins the list with the delimiter. A list is
       only possible with splat variables from resources with a count