The apex court recognised that the Constitution guarantees marriage equality.
The order is seen as a big victory for the lesbian, gay, bisexual, and transgender (LGBT) community, who have been fighting for their rights for decades.
Harris, also gay, thanked Justice Anthony Kennedy and the Supreme Court on Twitter, said The Hollywood Reporter.
"Your opinion is profound, in more ways than you may know," he tweeted.
"Congratulations America!," "Vicious" co-stars Ian McKellen and Derek Jacobi said in a joint statement.
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"I honor those who have bravely and tirelessly fought to make this a right and not a privilege for everyone living under our flag, with a special shout out to those at the Human Rights Campaign. I am ready to dance at some same-sex weddings," stated actress Lena Dunham.
"Grey's Anatomy" director Shonda Rhimes tweeted, "#MarriageEquality!! A giant step towards our country being a better place to be!"
"I have waited decades for this day, and my heart is full of joy and my eyes wet with tears," George Takei wrote in a Facebook post.
"Love wins. Happy day, I'm very proud to be an American and proud of my brothers and sisters that fought the good fight. Now, let's fix the gun thing," said Michael Urie in a statement.
Singer Adam Lambert stated, "This day is a great celebration in the worldwide movement towards full civil rights for everyone, but we must remember that no one is fully free while others remain oppressed."