Jim McGrath said the former president and his wife, Barbara, attended the wedding last weekend of Helen Thorgalsen and Bonnie Clement because the two women are friends, CNN reported. He said the Bushes were at the wedding as private citizens.
"He signed the marriage license after an impromptu request from the couple," McGrath said.
Thorgalsen later posted a picture of Bush signing the document on Facebook with the caption "Getting our marriage license witnessed!"
Same-sex marriage has been legal in Maine since voters approved a referendum last year, reversing an earlier vote.