diff --git a/catalog.go b/catalog.go
index da3c6bb..a4d7bc6 100644
--- a/catalog.go
+++ b/catalog.go
@@ -2,20 +2,36 @@ package stac
 
 import (
 	"encoding/json"
+	"fmt"
+	"regexp"
 
 	"github.com/mitchellh/mapstructure"
 )
 
 type Catalog struct {
-	Version     string   `json:"stac_version"`
-	Id          string   `json:"id"`
-	Title       string   `json:"title,omitempty"`
-	Description string   `json:"description"`
-	Links       []*Link  `json:"links"`
-	ConformsTo  []string `json:"conformsTo,omitempty"`
+	Version     string      `json:"stac_version"`
+	Id          string      `json:"id"`
+	Title       string      `json:"title,omitempty"`
+	Description string      `json:"description"`
+	Links       []*Link     `json:"links"`
+	ConformsTo  []string    `json:"conformsTo,omitempty"`
+	Extensions  []Extension `json:"-"`
 }
 
-var _ json.Marshaler = (*Catalog)(nil)
+var (
+	_ json.Marshaler   = (*Catalog)(nil)
+	_ json.Unmarshaler = (*Catalog)(nil)
+)
+
+var catalogExtensions = newExtensionRegistry()
+
+func RegisterCatalogExtension(pattern *regexp.Regexp, provider ExtensionProvider) {
+	catalogExtensions.register(pattern, provider)
+}
+
+func GetCatalogExtension(uri string) Extension {
+	return catalogExtensions.get(uri)
+}
 
 func (catalog Catalog) MarshalJSON() ([]byte, error) {
 	collectionMap := map[string]any{
@@ -34,5 +50,63 @@ func (catalog Catalog) MarshalJSON() ([]byte, error) {
 		return nil, decodeErr
 	}
 
+	extensionUris := []string{}
+	lookup := map[string]bool{}
+
+	for _, extension := range catalog.Extensions {
+		if err := extension.Encode(collectionMap); err != nil {
+			return nil, err
+		}
+		uris, err := GetExtensionUris(collectionMap)
+		if err != nil {
+			return nil, err
+		}
+		uris = append(uris, extension.URI())
+		for _, uri := range uris {
+			if !lookup[uri] {
+				extensionUris = append(extensionUris, uri)
+				lookup[uri] = true
+			}
+		}
+	}
+
+	SetExtensionUris(collectionMap, extensionUris)
 	return json.Marshal(collectionMap)
 }
+
+func (catalog *Catalog) UnmarshalJSON(data []byte) error {
+	collectionMap := map[string]any{}
+	if err := json.Unmarshal(data, &collectionMap); err != nil {
+		return err
+	}
+
+	decoder, decoderErr := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
+		TagName: "json",
+		Result:  catalog,
+	})
+	if decoderErr != nil {
+		return decoderErr
+	}
+
+	if err := decoder.Decode(collectionMap); err != nil {
+		return err
+	}
+
+	extensionUris, err := GetExtensionUris(collectionMap)
+	if err != nil {
+		return err
+	}
+
+	for _, uri := range extensionUris {
+		extension := GetCatalogExtension(uri)
+		if extension == nil {
+			continue
+		}
+		if err := extension.Decode(collectionMap); err != nil {
+			return fmt.Errorf("decoding error for %s: %w", uri, err)
+		}
+		catalog.Extensions = append(catalog.Extensions, extension)
+	}
+
+	return nil
+}
diff --git a/catalog_test.go b/catalog_test.go
index ab88ab7..c225c77 100644
--- a/catalog_test.go
+++ b/catalog_test.go
@@ -2,8 +2,10 @@ package stac_test
 
 import (
 	"encoding/json"
+	"regexp"
 	"testing"
 
+	"github.com/mitchellh/mapstructure"
 	"github.com/planetlabs/go-stac"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -38,3 +40,135 @@ func TestCatalogMarshal(t *testing.T) {
 
 	assert.JSONEq(t, expected, string(data))
 }
+
+const (
+	extensionAlias   = "test-catalog-extension"
+	extensionUri     = "https://example.com/test-catalog-extension/v1.0.0/schema.json"
+	extensionPattern = `https://example.com/test-catalog-extension/v1\..*/schema.json`
+)
+
+type CatalogExtension struct {
+	RequiredNum  float64 `json:"required_num"`
+	OptionalBool *bool   `json:"optional_bool,omitempty"`
+}
+
+var _ stac.Extension = (*CatalogExtension)(nil)
+
+func (*CatalogExtension) URI() string {
+	return extensionUri
+}
+
+func (e *CatalogExtension) Encode(catalogMap map[string]any) error {
+	extendedProps := map[string]any{}
+	encoder, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
+		TagName: "json",
+		Result:  &extendedProps,
+	})
+	if err != nil {
+		return err
+	}
+	if err := encoder.Decode(e); err != nil {
+		return err
+	}
+	catalogMap[extensionAlias] = extendedProps
+	return nil
+}
+
+func (e *CatalogExtension) Decode(catalogMap map[string]any) error {
+	extendedProps, present := catalogMap[extensionAlias]
+	if !present {
+		return nil
+	}
+
+	decoder, err := mapstructure.NewDecoder(&mapstructure.DecoderConfig{
+		TagName: "json",
+		Result:  e,
+	})
+	if err != nil {
+		return err
+	}
+	return decoder.Decode(extendedProps)
+}
+
+func TestExtendedCatalogMarshal(t *testing.T) {
+	stac.RegisterCatalogExtension(
+		regexp.MustCompile(extensionPattern),
+		func() stac.Extension {
+			return &CatalogExtension{}
+		},
+	)
+
+	catalog := &stac.Catalog{
+		Description: "Test catalog with extension",
+		Id:          "catalog-id",
+		Extensions: []stac.Extension{
+			&CatalogExtension{
+				RequiredNum: 42,
+			},
+		},
+		Links:   []*stac.Link{},
+		Version: "1.2.3",
+	}
+
+	data, err := json.Marshal(catalog)
+	require.NoError(t, err)
+
+	expected := `{
+		"type": "Catalog",
+		"description": "Test catalog with extension",
+		"id": "catalog-id",
+		"test-catalog-extension": {
+			"required_num": 42
+		},
+		"links": [],
+		"stac_extensions": [
+			"https://example.com/test-catalog-extension/v1.0.0/schema.json"
+		],
+		"stac_version": "1.2.3"
+	}`
+
+	assert.JSONEq(t, expected, string(data))
+}
+
+func TestExtendedCatalogUnmarshal(t *testing.T) {
+	stac.RegisterCatalogExtension(
+		regexp.MustCompile(extensionPattern),
+		func() stac.Extension {
+			return &CatalogExtension{}
+		},
+	)
+
+	data := []byte(`{
+		"type": "Catalog",
+		"description": "Test catalog with extension",
+		"id": "catalog-id",
+		"test-catalog-extension": {
+			"required_num": 100,
+			"optional_bool": true
+		},
+		"links": [],
+		"stac_extensions": [
+			"https://example.com/test-catalog-extension/v1.0.0/schema.json"
+		],
+		"stac_version": "1.2.3"
+	}`)
+
+	catalog := &stac.Catalog{}
+	require.NoError(t, json.Unmarshal(data, catalog))
+
+	b := true
+	expected := &stac.Catalog{
+		Description: "Test catalog with extension",
+		Id:          "catalog-id",
+		Extensions: []stac.Extension{
+			&CatalogExtension{
+				RequiredNum:  100,
+				OptionalBool: &b,
+			},
+		},
+		Links:   []*stac.Link{},
+		Version: "1.2.3",
+	}
+
+	assert.Equal(t, expected, catalog)
+}