The president of the Citizens for Equal Rights movement in Hawaii says the civil union law has ended a decades-long struggle for same sex marriage rights.
Four gay couples were the first to have their civil union cermonies, just after midnight on New Year's Day when the law took effect.
Tambry Young says some of the couples have waited years to get their relationship legally recognised, making the change an emotional moment for everyone there.
"We had people just from the GLBT community, as well as straight allies that have been with us throughout this movement, as well as family members of the couples, and everyone was so thrilled that they could be part of this historical event in Hawaii."
Tambry Young says the US state has struggled with the issue of legally recognising civil unions since the early nineties.
However, Governor Neil Abercrombie, a Democrat, signed the bill into law last February, after the previous Governor Linda Lingle, a Republican, had vetoed the bill.