The Spanish centre-back becomes Arsenal's first signing since Mikel Arteta's appointment as head coach.
Arsenal have completed the signing of Pablo Mari on a loan deal with the option to make the move permanent at the end of the season.
The deal for the Spaniard is worth up to £12m, with Arsenal paying a £4m loan fee upfront while having the option to make the move permanent for £8m in the summer.
"When I spoke to my agent, it was a big thing that Arsenal were interested in me," Mari told the Arsenal website.
"This is one of the best clubs in the world, so I'm absolutely delighted to be joining. I'm really looking forward to playing in this jersey and helping my team.
Flamengo already confirmed the departure of Pablo Mari earlier in the day. They didn’t make any mention of the Gunners, only thanking the defender for everything and adding “the time has come to say goodbye”.
The 26-year-old Spanish centre-back played in Brazil for Flamengo. He won the Copa Libertadores with the team last year, the highest level of competition in South American football. They also won the Brazilian Serie A.
On an individual level, he made the Campeonato Brasileiro Serie A Team of the Year in 2019.