diff --git a/config/interpolate_funcs.go b/config/interpolate_funcs.go
index 532dd50d12..4859624f7b 100644
--- a/config/interpolate_funcs.go
+++ b/config/interpolate_funcs.go
@@ -548,17 +548,32 @@ func interpolationFuncSplit() ast.Function {
 // dynamic lookups of map types within a Terraform configuration.
 func interpolationFuncLookup(vs map[string]ast.Variable) ast.Function {
 	return ast.Function{
-		ArgTypes:   []ast.Type{ast.TypeMap, ast.TypeString},
-		ReturnType: ast.TypeString,
+		ArgTypes:     []ast.Type{ast.TypeMap, ast.TypeString},
+		ReturnType:   ast.TypeString,
+		Variadic:     true,
+		VariadicType: ast.TypeString,
 		Callback: func(args []interface{}) (interface{}, error) {
+			defaultValue := ""
+			defaultValueSet := false
+			if len(args) > 2 {
+				defaultValue = args[2].(string)
+				defaultValueSet = true
+			}
+			if len(args) > 3 {
+				return "", fmt.Errorf("lookup() takes no more than three arguments")
+			}
 			index := args[1].(string)
 			mapVar := args[0].(map[string]ast.Variable)
 
 			v, ok := mapVar[index]
 			if !ok {
-				return "", fmt.Errorf(
-					"lookup failed to find '%s'",
-					args[1].(string))
+				if defaultValueSet {
+					return defaultValue, nil
+				} else {
+					return "", fmt.Errorf(
+						"lookup failed to find '%s'",
+						args[1].(string))
+				}
 			}
 			if v.Type != ast.TypeString {
 				return "", fmt.Errorf(
diff --git a/config/interpolate_funcs_test.go b/config/interpolate_funcs_test.go
index 5b99281f50..da12c22f56 100644
--- a/config/interpolate_funcs_test.go
+++ b/config/interpolate_funcs_test.go
@@ -713,12 +713,33 @@ func TestInterpolateFuncLookup(t *testing.T) {
 				true,
 			},
 
+			// Supplied default with valid key
+			{
+				`${lookup(var.foo, "bar", "")}`,
+				"baz",
+				false,
+			},
+
+			// Supplied default with invalid key
+			{
+				`${lookup(var.foo, "zip", "")}`,
+				"",
+				false,
+			},
+
 			// Too many args
 			{
-				`${lookup(var.foo, "bar", "baz")}`,
+				`${lookup(var.foo, "bar", "", "abc")}`,
 				nil,
 				true,
 			},
+
+			// Non-empty default
+			{
+				`${lookup(var.foo, "zap", "xyz")}`,
+				"xyz",
+				false,
+			},
 		},
 	})
 }
diff --git a/website/source/docs/configuration/interpolation.html.md b/website/source/docs/configuration/interpolation.html.md
index 85d2708ebe..3b414db7b0 100644
--- a/website/source/docs/configuration/interpolation.html.md
+++ b/website/source/docs/configuration/interpolation.html.md
@@ -160,9 +160,11 @@ The supported built-in functions are:
       * `${length(split(",", "a,b,c"))}` = 3
       * `${length("a,b,c")}` = 5
 
-  * `lookup(map, key)` - Performs a dynamic lookup into a mapping
+  * `lookup(map, key [, default])` - Performs a dynamic lookup into a mapping
       variable. The `map` parameter should be another variable, such
-      as `var.amis`.
+      as `var.amis`. If `key` does not exist in `map`, the interpolation will
+      fail unless you specify a third argument, `default`, which should be a
+      string value to return if no `key` is found in `map.
 
   * `lower(string)` - Returns a copy of the string with all Unicode letters mapped to their lower case.