The 29-year-old Argentina international joins from FC Metalist on a one-year loan deal with the option to make the transfer permanent at the end of the season
The 29-year-old midfielder, who joined Metalist from Napoli in 2011, spent last season on loan at Atletico Madrid, winning the Liga title with Diego Simeone’s side.
And on Monday afternoon Besiktas confirmed his arrival via a statement on their official website.
"I was told good things about Besiktas and their fans," Sosa said. "I am very happy to be here."
The Turkish giants will pay a €1.4 million loan fee, with the option to make the transfer permanent for an additional €2 million.
Sosa was a late addition to the Argentina squad at World Cup 2014 and, following injuries to the likes of Angel Di Maria, became a regular in the side that eventually ended runners-up after going down 1-0 to Germany in the final.
He joins fellow recent arrivals Demba Ba, Ramon Motta and Gokhan Tore as the club prepare to challenge for what would be their first league title since 2009.
Slaven Bilic’s side opened their Turkish Super Lig campaign last week with a 1-0 victory over Mersin İdmanyurdu.