It happened around 1:30am at a home in the 11000 block of Westway at Gaston in northeast Houston.
Deputies said a man went to his ex-girlfriend's home, thinking she was asleep alone.
He knocked on her bedroom window and she opened her curtain. That's when officials said he saw her new boyfriend in bed.
The ex-boyfriend reportedly told the woman he was going to get his gun and he returned 15 minutes later. He barged into the home with a pistol and got into a fight with the other man, officials said.
Deputies said the new boyfriend also had a gun. He pulled it out and shot the man once in the chest during the scuffle.
The shooting victim was transported to Ben Taub General Hospital in serious but stable condition. It's unknown if the shooter will face charges.