Much to the disappointment of the Barclays Center crowd, all three judges scored the fight 115-113 in favour of Thurman.
Thurman defended his title and improved to 27-0, while Porter is now 26-2-1 but may yet get a rematch.
"I'd give him a rematch. It was a great fight. He was a great opponent," Thurman said during an in-ring interview.
The Americans went at each other from the outset and they landed decent blows during the opening four rounds.
The entertaining and close fight could have gone either way, but it was Thurman who retained to heavy boos.
Asked if he felt he won the bout, Porter replied: "I do. I do, but I'm satisfied because the competitor [in Thurman] came out tonight."
He added: "At the end of the day, I'm blessed to have these fans right now. We worked hard. He's a great champion."