Watt has been on crutches since he broke his left leg on Oct. 8.
It was the first public appearance for Watt, who is a three-time Defensive Player of the Year, since he sustained the season-ending injury against the Kansas City Chiefs.
Watt, who started a fundraiser that brought in more than $37 million for Hurricane Harvey relief efforts, received a huge ovation as he was introduced.
He wore a bright orange Astros jersey with his name and No. 99, athletic shorts and had a large black brace on his injured leg.
He made his way out to the mound before handing one of his crutches to girlfriend and Houston Dash soccer player Kealia Ohai. He then launched a pitch that was a bit outside to Dallas Keuchel.
He made a hand gesture as if to indicate that his pitch was so-so before getting his second crutch back and moving toward Keuchel. The two embraced and talked for a few seconds before the lefty ace signed the ball for him.
Information from the Associated Press was used in this report.