He expressed his views in a hand written message on his website which has proved extremely popular in Cambodia.
The king said that as a "liberal democracy", Cambodia should allow "marriage between man and man… or between woman and woman."
He said he had respect for homosexual and lesbians and said they were as they were because God loved a "wide range of tastes."
Sihanouk, who is currently in Beijing for medical treatment, also said that transvestites should be "accepted and well-treated in our national community."
Such views are not widespread in Cambodia, but the king is hugely revered, although he is a constitutional monarch and has no executive powers.
San Francisco has issued more than 2,800 marriage licences to gay couples in the past week amid a growing debate in the US over whether such unions should be allowed.