Tuesday's veto came on the final day that Republican Gov. Linda Lingle had to either sign or veto the contentious bill. The Hawaii Legislature had approved it in late April.
Had Lingle not vetoed it, the measure would have granted gay and lesbian couples the same rights and benefits that the state provides to married couples.
It also would have made Hawaii one of six states that essentially grant the rights of marriage to same-sex couples without authorizing marriage itself. Five other states and the District of Columbia permit same-sex marriage.
Earlier Tuesday, supporters waved rainbow flags in front of the state Capitol. Opponents prayed in the hallways for a veto.
