The Suquamish tribe in Washington state has approved same sex marriages:
“This spring, a young woman stood up at the tribe’s annual meeting on its reservation here on Puget Sound and asked it to formally approve same-sex marriage. The response from the 300 or so people present was an enthusiastic “yes” in a voice vote. There was no audible dissent. Then, after another, smaller meeting (still no opposition) and a little work by the tribal attorney, the tribal council voted unanimously this month to approve same-sex marriage. No court fights. No ballot measures. No billionaires behind the scenes.”
At least one other tribe has approved it as well, according to the article written by William Yardley in the NY Times. Many other tribes do not make their laws public.