U.S. District Judge Michael McShane threw out the voter-approved ban Monday.
State officials have said they'd be prepared to carry out same-sex marriages almost immediately, and couples lined up outside the county clerk's office in Portland in anticipation of the decision.
Four gay and lesbian couples brought suit arguing Oregon's marriage laws unconstitutionally discriminate against same-sex couples and exclude them from a fundamental right.
State officials refused to defend the ban, and McShane earlier denied a request by the National Organization for Marriage to intervene on behalf of its Oregon members.
An appeals court Monday morning refused the group's request for an emergency stay of McShane's decision.