The transfer was announced well after the 22:00 GMT deadline but Chelsea had submitted the paperwork to the Premier League in time after the former Arsenal striker arrived in London late on deadline day.
Barcelona said they reached an agreement where Chelsea paid 12 million euros to bolster their front line with Aubameyang, who previously thrived under manager Thomas Tuchel at Borussia Dortmund.
"I'm really happy. It's an honour to be part of this team and I can't wait to start. I have some unfinished business with the Premier League so it's good to be back," Aubameyang said in a statement.
Aubameyang scored 13 goals in 24 games for Barcelona, but was surplus to requirements after they signed Poland striker Robert Lewandowski from Bayern Munich and needed to offload players to comply with LaLiga's financial rules.
Aubameyang is no stranger to the Premier League having spent four years at Chelsea's London rivals Arsenal, scoring 92 goals in all competitions.
He was the league's joint top scorer in the 2018-19 season and led Arsenal to the FA Cup title in the following campaign.
However, the Gabon international had a fall-out with Arsenal and their manager Mikel Arteta last season over disciplinary issues. He was first stripped of the captaincy before having his contract terminated ahead of a February move to Barcelona.
Chelsea have been playing without a recognised striker after Romelu Lukaku moved back to Inter Milan on loan in the close season while Timo Werner returned to RB Leipzig.
Chelsea also announced the signing of Swiss international Denis Zakaria from Juventus on a season-long loan, with the English club having the option to make the central midfielder's stay permanent at the end of the season.
Midfield reinforcements were the need of the hour at Chelsea with Tuchel saying N'Golo Kante's hamstring injury is set to rule him out for several weeks while Ruben Loftus-Cheek is also sidelined.
The London club, 10th after five games, have signed a number of high-profile players in the transfer window, including Raheem Sterling and Kalidou Koulibaly.
Meanwhile, full back Marcos Alonso, who joined Chelsea in 2016 for around £23 million and had one year left on his contract, has joined Barcelona as part of the deal according to media reports.
