Companies including Microsoft Corp, Google Inc, Starbucks Corp and Pfizer Inc are among those that joined the brief. Others included Aetna Inc, Amazon.com, Inc and Citigroup Inc.

Thomson Reuters Corp is another signatory. The Reuters news agency is part of Thomson Reuters.

The companies want the Supreme Court to strike down a key provision of the federal Defense of Marriage Act (DOMA) that defines marriage as a union between a man and a woman.

Separately, lawyers representing another group of employers, including some of the same companies, had said already that they planned to file a brief on Thursday in a related case that questions a California law that bans gay marriage.