The 27-year-old Johnson won his second ATP Tour title, and became the seventh American to win the event since it moved to Houston in 2001. He also won a grass-court title last summer in Nottingham, England.
Johnson was hobbled by cramps serving at the end of the third set, but — coaxing a backhand return error from Bellucci on a serve he barely hit — was able to get off the court and receive a brisk right-thigh massage from an ATP trainer. Players aren't allowed timeouts for cramps. Johnson appeared fine in the tiebreaker, the first to decide the championship in tournament history.
"I was in deep trouble. 'This stinks.' But sometimes you get lucky," said Johnson, seeded fourth. "My body has physically run out of gas this week. Coming in from Australia (where the U.S. lost a Davis Cup quarterfinal last weekend), dealing with all the ups and downs, I was kind of behind the 8-ball.
"A lot goes into cramping. For me today, it was the nerves of trying to close out the title. I haven't been there enough to feel very comfortable."
Johnson beat two former champions, Fernando Verdasco (2014) in the quarterfinals and American Davis Cup teammate Jack Sock (2015) in the semifinals, to reach his third career final.
The 29-year-old Bellucci, seeded eighth, had won four three-set matches in four days before falling in the final. He was trying to win his fifth Tour title. The previous four had also come on clay.
"Both players were very tired in the third set and we were missing more shots than before," Bellucci said. "I'm happy for my week but today I am disappointed. I (thought) I could win because I was a little better than him physically. He deserved it more than me."
Johnson won NCAA singles titles for the University of Southern California in 2011 and 2012.