In his long-awaited ruling delivered late on Tuesday, Circuit Judge Kevin Chang found the state of Hawaii was in violation of sexual discrimination provisions in its own constitution by witholding marriage licenses from lesbians and gays.
Judge Chang ruled the state had failed to demonstrate a "compelling interest" as to why gays ought not be allowed to marry. The state had attempted to argue that allowing gay marriage in Hawaii might harm the welfare of any children homosexual couples tried to raise.
The decision is a milestone for supporters of gay marriage and for the three homosexual couples who first introduced the case five years ago. One of the plaintiffs, Nina Baehr, told reporters: "People told us we would never get this far in the courts, but when we heard the news there were tears in my eyes. I thought I would cry if we lost, but we cried for winning".
For conservative critics it will provide a fresh rallying-cry for renewed efforts to block what they see as a conspiracy by gay activists to subvert marriage. Robert Knight of the Family Research Council called the ruling an "outrage".
The holding of gay marriages in Hawaii may be delayed, however, as the state considers appealing to the State Supreme Court. Such an appeal, which is highly likely, could take most of next year to complete.
It is doubtful that the Supreme Court, which gave a provisional ruling in favour of gay marriage in the same case in 1993, would overturn Tuesday's ruling. In that case, it is probable that Hawaii would finally begin to issue marriage licences to gays and lesbians before the end of next year.
For Hawaii, it is likely to mean a flood of gays to the islands seeking to realise their dreams of marriage.
The political and legal battle that is already under way is focused on the implications of the ruling for the rest of the Union. In theory, the "full faith and credit" provisions of the US Constitution obliges every state to recognise the laws of others.
Last September, however, President Bill Clinton, with the election looming, signed the "Defence of Marriage Act" that invites states to refuse to recognise gay couples and serves to deny gays financial benefits extended to heterosexual married couples.