Mari, 26, moved to the Premier League side in January from Flamengo, who he helped win the Brazilian Serie A title and Copa Libertadores last year.
The Spanish centre-back played three times for Arsenal before his season was ended prematurely by an ankle injury.
Portugal full-back Soares, 28, who initially joined on a short-term loan deal from Southampton, made five league appearances for Mikel Arteta's side last season.
Arsenal have been linked with Lille centre back Gabriel Magalhaes as Arteta looks for more defensive cover ahead of the league campaign, which begins on September 12.
They also have defender William Saliba returning from his loan spell at Saint-Etienne.
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France youth coach Jean-Luc Vannuchi, who coached Saliba with France's Under-18s, believes Arsenal are getting a valuable asset back among their ranks, comparing him to Virgil van Dijk and Rafael Varane.
"He is a mix of the two styles," Vannuchi told The Telegraph. "With Varane because of his speed and power, and with Van Dijk it is the interceptions, the positioning on the pitch."