"If you like it you should be able to put a ring on it. #WeWillUnite4MarriageEquality," read the note.
The singer's pledge to marriage equality came hours after the first day of arguments on Proposition 8, California's controversial ban on gay marriage, began before the Supreme Court.
Beyoncé and her husband, Jay Z, are long-time supporters of marriage equality. Last spring, the rapper endorsed President Barack Obama's gay-marriage advocacy calling it "the right thing to do."