diff --git a/bip-0138.md b/bip-0138.md
index d9ceedb2..e4482b04 100644
--- a/bip-0138.md
+++ b/bip-0138.md
@@ -556,6 +556,12 @@ Implementations SHOULD encode and decode the backup using Base64 (RFC 4648).[^ps
     PSBT (BIP174) is commonly exchanged as a Base64 string, so wallet software
     likely already supports this representation.
 
+### File Extension
+
+Backups saved to a file SHOULD use the `.bip138` file extension, whether the file
+contains the raw binary encoding or its Base64 text representation. A Base64 backup
+MAY instead use the `.txt` extension.
+
 ## Rationale
 
 See footnotes throughout the specification for design rationale.