Thomas, 30, is a Ryder Cup veteran but has struggled this year, missing cuts at the Masters, US Open and The Open and failing to make the playoffs for the first time since 2014.
"Justin Thomas is one of the most talented players on the PGA Tour in my opinion," Johnson said.
"He has been, without question, the heart and soul of US Ryder Cup teams, our emotional leader.
"Overall [he has] a fantastic Ryder Cup record. His passion for the Ryder Cup is very evident. In my mind he is born for this and you just don't leave JT at home."
Two-time major winner Thomas, who carded a round of 81 at the US Open in June and 82 at The Open in July, said he was "very, very fortunate" to be picked.
"You can want something too bad," he added. "I'm sure all of us have tried to win golf tournaments too much and have tried to force the issue, and there were potentially times this season I did.
"Zach has been very vocal and made it sound great that we are all equal on this team. I feel I'm just as important as everybody else and my team-mates have voiced that the same way."
Koepka, who is the only LIV Golf player on the team, could only earn qualifying points towards the Ryder Cup at the major championships because the PGA of America did not count LIV Golf events for their rankings.
"A five-time major champion, he is built in my mind for the biggest of stages and there's no bigger stage than the Ryder Cup," said Johnson.
USA are the holders of the Ryder Cup after a record 19-9 victory in Wisconsin in 2021.
But they have not won on European soil for 30 years, their last victory on the continent coming at the Belfry in Warwickshire in 1993 with a 15-13 win.
European qualification concludes on 3 September after the European Masters at Crans-sur-Sierre in Switzerland.
As with the US, the top six European players in the standings will qualify and captain Luke Donald has six picks. He will announce the team on 4 September.
Rory McIlroy, Jon Rahm, Viktor Hovland and Tyrrell Hatton have already secured qualification spots.
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