Bush and wife Barbara attended last weekend's nuptials of Bonnie Clement and Helen Thorgalsen, American commercial broadcast television and radio network NBC reported citing a spokesperson for the former president.
The newlyweds posted on Twitter a photo of Bush signing their marriage licence as a witness.
Thorgalsen and Clement own a store in Kennebunk, Maine, near the Bush family's compound.
While gay marriage was not a political issue during G.H.W. Bush's 1989-1993 administration, son George W. Bush expressed opposition to same-sex unions during his eight years in the White House.
In 2004, George W. Bush endorsed the idea of a constitutional amendment banning gay marriage, even though his vice president, Dick Cheney, is the father of an openly lesbian daughter and a supporter of the right of gays and lesbians to wed.
The younger Bush's wife, Laura Bush, has spoken in favour of same-sex unions.