Messi arrived at Le Bourget airport to the north of the French capital around 3:30 pm (1330 GMT) and was greeted by hundreds of PSG supporters who had gathered hoping to catch a glimpse of their new signing.
Joan Laporta said signing Messi to a new contract did not depend on the outcome of an audit the club is carrying out to find out the exact state of its finances.
In a statement, Barca said that Aguero will become their player once his contract with Manchester City expires at the end of next month, adding that there is a buy-out clause 'set at 100 million euros ($122 million)'.
Gabriel Paulista headed Valencia in front early in the second half before Messi missed a penalty, only to smash in from the resulting scramble.
Messi tied Xavi's 767 games in all competitions for Barca and marked yet another milestone with a brilliant brace, a curling effort in the first half flying in off the crossbar at Camp Nou before a late shot nestled in the corner.
The Dutch international has long been linked with Barcelona and a reunion with his former national team boss Ronald Koeman.
That remains the nightmare scenario for Barcelona fans, even if their new president Joan Laporta announced following his election victory on Sunday that he was "convinced he wants to stay".
Laporta was last president at Barca between 2003 and 2010, which included overseeing the hugely successful appointment of Pep Guardiola as coach and the iconic treble-winning team of 2008/9.
Having trailed 2-0 from the first leg, Danish international Martin Braithwaite hit the crucial goal in the fifth minute of extra-time with a diving header off a Jordi Alba cross.
Spanish newspaper El Mundo controversially published details of Barcelona’s contract with Lionel Messi, showing the Argentinian will be paid up to 555 million euros over four years.
Speaking to Spanish TV channel La Sexta, Messi insisted that he was 'excited' to play under coach Ronald Koeman this season despite trying to leave the club where he has played his entire senior career last summer.
The Argentina forward scored Barca's third goal when he collected Pedri's backheel and struck the ball past goalkeeper Jordi Masip in the 65th minute.
The World Cup-winning Spanish midfielder, 36, said he planned to be back for his Japanese club Vissel Kobe next season after undergoing surgery in Barcelona on a ruptured tendon.
Messi struck with 14 minutes remaining of a lacklustre display from Barca at the Camp Nou which will do little to ease fan fears following their 3-0 hammering at the hands of Juventus midweek.
After the Portuguese forward arrived from Manchester United in 2009, El Clasico became synonymous with his rivalry with Barcelona captain Messi.
Ronald Koeman again rested Lionel Messi with qualification for the last 16 already assured, and Barca made it five wins from five in Group G with an impressive performance in Budapest.
Yannick Carrasco finished brilliantly from range after mistakes from Gerard Pique and Marc-Andre ter Stegen left the goal gaping but Barca, and a below-par Lionel Messi, failed to respond at the Wanda Metropolitano.
Lionel Messi won and then scored a penalty before the 17-year-old Ansu Fati contributed to two brilliant goals, volleying in one for himself and then setting up another for Philippe Coutinho.
