Jennings, 27, was taken off the field on a cart and to the hospital for observation. He will spend the night there while the team travels to Cincinnati.
The Marlins' organization tweeted: "Special thanks to the Pirates medical staff, athletic trainers and emergency personnel for their assistance."
Jennings struggled to maintain his balance as catcher Jarrod Saltalamacchia raced to the mound. Marlins trainer Sean Cunningham and manager Mike Redmond also rushed out there as Jennings dropped to a knee, clearly dazed. The stadium turned silent as fans and players from both dugouts displayed concern.
"He was responding to the questions Sean was asking him," Redmond said. "His eyes were open the whole time. He didn't lose consciousness. He was responding."
Jennings gave all the right answers, responding to where he was and what game in the series he was playing.
"The good thing is Sean got out there pretty quickly and talked to him," Saltalamacchia said. "He was aware. He knew where he was at."
The scary moment occurred with one out in the seventh inning of the Pirates' 7-2 victory. Pittsburgh's Jordy Mercer ripped a line drive up the middle that deflected off Jennings' head and straight up in the air.
After a few minutes, Jennings was assisted to a cart. He sat next to Cunningham and exited to cheers from the crowd. Jennings raised his right hand as if to inform the crowd that he was fine.
Pittsburgh is known for having a leading concussion specialist, which is encouraging for the left-hander.
As part of the play, Mercer actually was out, because the ball bounced in the air to shortstop Adeiny Hechavarria, who made the catch.
"It's somebody's career, it's somebody's family, they're trying to put food on the table for their family as well," Mercer said. "You don't want anything like that. I had the best seat in the house, basically, and it wasn't pretty. I'm going to keep him in my prayers."
The Marlins haven't made a roster move yet, but Jennings is expected to go on the disabled list.
For Redmond, the incident brought back memories of 2003 when the Marlins were in Boston. Reliever Kevin Olsen was struck on the side of the head by a Todd Walker line drive. Redmond was the catcher that night.
The injury also is a reminder of how vulnerable pitchers are to balls that can come back at them at 100 mph. In the past few years, Brandon McCarthy (then with Arizona), Alex Cobb (Rays) and J.A. Happ (Blue Jays) received head injuries.
"It's definitely scary," Saltalamacchia said. "I've never been on the field when something like that's happened. I saw Cobb go through it with the Rays. It's scary. It goes past baseball. You start to worry about life. He's got a wife and kid. Any time you get hit in the head, it's scary."