diff --git a/marshal.go b/marshal.go
index e664a96..81d6997 100644
--- a/marshal.go
+++ b/marshal.go
@@ -26,6 +26,7 @@ type marshalState struct {
 	schemaComments       bool
 	seenStructs          map[reflect.Type]bool
 	allowExtra           bool
+	defaultTransformer   func(string) string
 }
 
 // Create a shallow clone with schema overridden.
@@ -84,6 +85,13 @@ func WithSchemaComments(v bool) MarshalOption {
 	}
 }
 
+// WithDefaultTransformer allows custom processing of default values in struct tags.
+func WithDefaultTransformer(transformer func(string) string) MarshalOption {
+	return func(options *marshalState) {
+		options.defaultTransformer = transformer
+	}
+}
+
 func asSchema() MarshalOption {
 	return func(options *marshalState) {
 		options.schema = true
@@ -264,17 +272,17 @@ func fieldToAttr(field field, tag tag, opt *marshalState) (*Attribute, error) {
 	if err != nil {
 		return nil, err
 	}
-	attr.Default, err = defaultValueFromTag(field, tag.defaultValue)
+	attr.Default, err = defaultValueFromTag(field, tag.defaultValue, opt)
 	if err != nil {
 		return nil, err
 	}
 	attr.Optional = (tag.optional || attr.Default != nil) && opt.schema
-	attr.Enum, err = enumValuesFromTag(field, tag.enum)
+	attr.Enum, err = enumValuesFromTag(field, tag.enum, opt)
 	return attr, err
 }
 
-func defaultValueFromTag(f field, defaultValue string) (Value, error) {
-	v, err := valueFromTag(f, defaultValue)
+func defaultValueFromTag(f field, defaultValue string, opt *marshalState) (Value, error) {
+	v, err := valueFromTag(f, defaultValue, opt)
 	if err != nil {
 		return nil, fmt.Errorf("error parsing default value: %v", err)
 	}
@@ -282,7 +290,7 @@ func defaultValueFromTag(f field, defaultValue string) (Value, error) {
 }
 
 // enumValuesFromTag parses the enum string from tag into a list of Values
-func enumValuesFromTag(f field, enum string) ([]Value, error) {
+func enumValuesFromTag(f field, enum string, opt *marshalState) ([]Value, error) {
 	if enum == "" {
 		return nil, nil
 	}
@@ -290,7 +298,7 @@ func enumValuesFromTag(f field, enum string) ([]Value, error) {
 	enums := strings.Split(enum, ",")
 	list := make([]Value, 0, len(enums))
 	for _, e := range enums {
-		enumVal, err := valueFromTag(f, e)
+		enumVal, err := valueFromTag(f, e, opt)
 		if err != nil {
 			return nil, fmt.Errorf("error parsing enum: %v", err)
 		}
@@ -302,11 +310,15 @@ func enumValuesFromTag(f field, enum string) ([]Value, error) {
 
 }
 
-func valueFromTag(f field, defaultValue string) (Value, error) {
+func valueFromTag(f field, defaultValue string, opt *marshalState) (Value, error) {
 	if defaultValue == "" {
 		return nil, nil // nolint: nilnil
 	}
 
+	if opt != nil && opt.defaultTransformer != nil {
+		defaultValue = opt.defaultTransformer(defaultValue)
+	}
+
 	k := f.v.Kind()
 	if k == reflect.Ptr {
 		k = f.v.Type().Elem().Kind()
@@ -372,7 +384,7 @@ func valueFromTag(f field, defaultValue string) (Value, error) {
 				t: reflect.StructField{},
 				v: reflect.New(valueType),
 			}
-			val, err := defaultValueFromTag(valueField, v)
+			val, err := defaultValueFromTag(valueField, v, opt)
 			if err != nil {
 				return nil, fmt.Errorf("error parsing map %q into value, %v", v, err)
 			}
@@ -406,7 +418,7 @@ func valueFromTag(f field, defaultValue string) (Value, error) {
 			v: reflect.New(valueType),
 		}
 		for _, item := range list {
-			value, err := defaultValueFromTag(valueField, item)
+			value, err := defaultValueFromTag(valueField, item, opt)
 			if err != nil {
 				return nil, fmt.Errorf("error applying %q to list: %v", item, err)
 			}
diff --git a/unmarshal.go b/unmarshal.go
index 3b4b259..d61cf74 100644
--- a/unmarshal.go
+++ b/unmarshal.go
@@ -129,7 +129,7 @@ func unmarshalEntries(v reflect.Value, entries []Entry, opt *marshalState) error
 				return fmt.Errorf("missing required attribute %q", tag.name)
 			}
 			// apply defaults here as there's no value for this field
-			v, err := defaultValueFromTag(field, tag.defaultValue)
+			v, err := defaultValueFromTag(field, tag.defaultValue, opt)
 			if err != nil {
 				return err
 			}
@@ -297,7 +297,7 @@ func checkEnum(v Value, f field, enum string) error {
 	case reflect.Map, reflect.Struct, reflect.Array, reflect.Slice:
 		return fmt.Errorf("enum on map, struct, array and slice are not supported on field %q", f.t.Name)
 	default:
-		enums, err := enumValuesFromTag(f, enum)
+		enums, err := enumValuesFromTag(f, enum, nil)
 		if err != nil {
 			return err
 		}
diff --git a/unmarshal_test.go b/unmarshal_test.go
index cb22765..d8bd5d4 100644
--- a/unmarshal_test.go
+++ b/unmarshal_test.go
@@ -3,6 +3,7 @@ package hcl
 import (
 	"fmt"
 	"net"
+	"os"
 	"reflect"
 	"sort"
 	"strconv"
@@ -1102,3 +1103,87 @@ func TestUnmarshallInterfaces(t *testing.T) {
 	}
 	runTests(t, tests)
 }
+
+func TestWithDefaultTransformer(t *testing.T) {
+	expandVars := func(vars map[string]string) func(string) string {
+		return func(defaultValue string) string {
+			return os.Expand(defaultValue, func(key string) string {
+				if val, ok := vars[key]; ok {
+					return val
+				}
+				return os.Getenv(key)
+			})
+		}
+	}
+
+	t.Setenv("FALLBACK_VAR", "fallback-value")
+
+	tests := []test{
+		{
+			name: "shell-style variable expansion",
+			hcl:  ``,
+			dest: struct {
+				BaseURL string `hcl:"base-url,optional" default:"${BASE_URL}/api/v1"`
+			}{
+				BaseURL: "https://api.example.com/api/v1",
+			},
+			options: []MarshalOption{WithDefaultTransformer(expandVars(map[string]string{
+				"BASE_URL": "https://api.example.com",
+			}))},
+		},
+		{
+			name: "custom template syntax",
+			hcl:  ``,
+			dest: struct {
+				Message string `hcl:"message,optional" default:"Hello {{NAME}}!"`
+			}{
+				Message: "Hello World!",
+			},
+			options: []MarshalOption{WithDefaultTransformer(func(s string) string {
+				if s == "Hello {{NAME}}!" {
+					return "Hello World!"
+				}
+				return s
+			})},
+		},
+		{
+			name: "string replacement",
+			hcl:  ``,
+			dest: struct {
+				Path string `hcl:"path,optional" default:"__HOME__/config"`
+			}{
+				Path: "/home/user/config",
+			},
+			options: []MarshalOption{WithDefaultTransformer(func(s string) string {
+				if s == "__HOME__/config" {
+					return "/home/user/config"
+				}
+				return s
+			})},
+		},
+		{
+			name: "prefix addition",
+			hcl:  ``,
+			dest: struct {
+				Topic string `hcl:"topic,optional" default:"events"`
+			}{
+				Topic: "prod.events",
+			},
+			options: []MarshalOption{WithDefaultTransformer(func(s string) string {
+				return "prod." + s
+			})},
+		},
+		{
+			name: "environment variable fallback",
+			hcl:  ``,
+			dest: struct {
+				Value string `hcl:"value,optional" default:"${FALLBACK_VAR}"`
+			}{
+				Value: "fallback-value",
+			},
+			options: []MarshalOption{WithDefaultTransformer(expandVars(map[string]string{}))},
+		},
+	}
+
+	runTests(t, tests)
+}