But the reality television host did laud Justice Anthony Kennedy, who led conservative opposition to the bill on the court.
"People should be proud of him. He really, really did stick up for his values," Trump said.
And Trump said he sensed Republicans were more motivated after the ruling, saying that there was noticeably more buzz at a fundraiser he attended with Romney the day the ruling was announced.
"The energy in that room was five times greater than it would have been a month ago," Trump said.
It was Trump's first time publicly addressing the fundraiser, a point of confusion last week after the Romney campaign initially indicated that it would not occur, and then refused to comment on it. Trump, however, accused the press of trying to undermine the GOP candidate.
"We had a big fundraiser that everybody said, 'Oh, they're not going to have the fundraiser'; the press said, 'The fundraiser's not going to happen, Trump's going to be embarrassed,' then all of a sudden we had a fundraiser that raised millions of dollars — it was a great success," Trump said. "It's amazing how the press can be so dishonest. They were told it was going to happen. It's amazing."