The former Italy international left hometown club Roma at the end of last season after learning his contract would not be renewed.
Revered at Stadio Olimpico, where he debuted in October 2001, the 36 year-old De Rossi will prolong his playing days at the equally iconic La Bombonera after signing a deal.
He arrives in Buenos Aires as a World Cup winner who won Coppa Italia twice and the Supercoppa Italiana with the Giallorossi.
De Rossi's phenomenal career in numbers:
616 - De Rossi played 616 times for Roma in all competitions, albeit still 170 short of the club's record appearance maker Francesco Totti.
61 - The midfielder has, however, featured in more Champions League/European Cup matches than any other Roma player. His tally of 61 is four more than Totti managed.
459 - In total, De Rossi featured in 459 Serie A games for Roma.
249 - With De Rossi in the side, Roma won 249 league matches. It lost on 99 occasions and had 111 draws.
63 - De Rossi scored 63 goals for Roma in his 18-year spell in the first-team – 43 of those were in Serie A, with 13 in European competitions.
117 - Only three players have won more Italy caps than De Rossi's 117. They are Gianluigi Buffon (176), Fabio Cannavaro (136) and Paolo Maldini (126).
21 - De Rossi found the net against 21 of the 40 teams he faced in Serie A. He also managed 21 international goals.
4 - Chievo was De Rossi's favourite opponent when it came to getting himself on the scoresheet, as the holding midfielder netted four times in 24 league appearances against them.
355 - Only two players in Roma's history – De Rossi and Totti – have made at least 500 appearances for the club. De Rossi also played alongside the forward 355 times in Serie A.