The 30-year-old has made a permanent switch to Mano Menezes' team, returning to a club where he enjoyed a 22-match loan spell in 2005/06.
Santos joined the Gunners from Fenerbahce two years ago for £6.8 million, but after an impressive first few months he struggled with injuries and failed to earn a regular place in the side.
The Brazil international fell down the pecking order at Emirates Stadium behind England left-back Kieran Gibbs and, following the deadline day arrival of Spaniard Nacho Monreal in January, was sent out on loan to Gremio in February.
He made 33 appearances in total for Arsenal, scoring three goals.
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- Mano Menezes