Can I register an arrangement of a public domain song?

Registering arrangements of public domain songs

Yes. It's important that you're working within the confines of 'public domain'. Public domain works are considered those where copyright has expired or author(s) explicitly put these works into public domain.

Example: you record an arrangement of Deck The Halls – a Christmas song in the public domain. You will be able to register this song as the arranger, but not the composer/author.

Please note that most PROs pay out performance royalties at a reduced rate for arrangements of public domain works.