The attacking duo, accompanied by Dani Alves and Javier Mascherano, have joined up with the rest of Luis Enrique's squad following an extended break after the World Cup
Argentina's run to the final was thanks in large parts to Barca's Messi and Mascherano, while Alves played a part in Brazil's fourth-placed finish, and new coach Luis Enrique now has the trio back available for selection in pre-season.
Neymar's back injury - which ruled him out of Brazil's World Cup semi-final defeat to Germany and the third-place play-off loss to Netherlands - will be evaluated by the Catalans' medical team but is believed to have fully recovered.
The returning players will train on Tuesday while the rest of La Liga's runners up in 2013-14 have been given the afternoon off.
Players who featured at the World Cup have been trickling back into Barca's squad for weeks since training recommenced under Luis Enrique on July 14, with those eliminated in the group stages being brought back first and the early knockout phase competitiors coming shortly afterwards.
Barca's pre-season schedule has been relatively low-key so far, with just a 1-0 win over Recreativo Huelva and a 1-1 draw with Nice in three weeks of preparations for 2014-15.
However, they have three more friendlies scheduled - against Napoli, HJK and Leon - before they kick off their Primera Division season at home to Elche on Sunday, August 24.
Following a season that ended with just the Supercopa to show for their efforts, Gerardo Martino's replacement, Luis Enrique, will be hoping to bring more notable successes back to Camp Nou this campaign.