The Argentine World Player of the Year, top scorer in Europe's elite club competition the past four seasons, had only been given the all-clear shortly before kickoff after damaging a hamstring in the quarter-final first leg 2-2 draw last week.
Coach Tito Vilanova decided to keep him in reserve but his hand was forced when Javier Pastore raced clear to give the Ligue 1 side a shock lead five minutes after half-time.
Messi replaced an ineffectual Cesc Fabregas in the 62nd minute and the buzz was palpable among almost 100,000 Barca fans packed into the giant arena on a balmy night in the Catalan capital.
Although struggling for fitness and lacking his customary zip, Messi's entrance immediately settled his team mates' nerves and restored their sense of urgency.
With Barca laying siege to the PSG goal and roared on by the home faithful, he fed David Villa who clipped a short pass to Pedro for his Spain team mate to lash a shot past Salvatore Sirigu from the edge of the area.
The visitors mounted several desperate attacks in a tense finale but Barca held on reasonably comfortably to advance on away goals and keep their bid for a third European crown in five years firmly on track.
"Messi made a big effort to be with us," Pedro said in an interview with Spanish television Canal Plus.
"He worked very hard this week and was able to help out the team when he came on," added the 25-year-old.
"We want to thank him, and say congratulations for everything he did to get himself ready for the game and to help the team. He's the number one.
"We felt his presence when he came on the pitch, he lifted us and we got the result."
Barca sporting Andoni Zubizarreta also paid tribute to Messi, who has been top scorer in the Champions League the past four seasons.
"We all know what Messi brings to the game, in football terms, in his ability to intimidate opponents, and for his ability to link up with others," the former Barca and Spain goalkeeper told Canal Plus.
"He wasn't at his best but he showed he could link up with others, like David [Villa], and helped get the equaliser."
QATARI FLAVOUR
In a match with a distinctly Qatari flavour - PSG are owned by the oil-and gas-rich Arab nation and Qatar Foundation is Barca's main sponsor - the visitors made a lively start.
Ezequiel Lavezzi latched onto a Sergio Busquets error and raced through the middle in the fourth minute but his control let him down and Valdes was able to clear.
The Spain keeper was called on again in the 24th minute when Zlatan Ibrahimovic sent Lavezzi clear and he saved well with an outstretched leg before palming a powerful Lucas header over the bar as PSG pressed for a goal.
Despite boasting nine Spain internationals in their starting line-up, Barca were looking unusually ragged with the normally impeccable Busquets misplacing several passes and full-back Daniel Alves fluffing a couple of crosses.
