Gago had already spent the first six months of the year in Buenos Aires on loan from Valencia to Velez Sarsfield but Boca stepped into bring the player back to the club where he made his first division debut in 2004.
"It's a nice feeling coming back to the club where I grew up as a person ... and now I want to enjoy this stage at Boca," the 27-year-old told a news conference.
"Boca always want to be (title) candidates, that's the goal. We have the obligation to fight for the title."
Boca had a poor 'Final' championship last season finishing one from bottom and will be gunning for the 'Inicial', the first championship of the 2013/14 season starting this weekend.
Gago joined Real Madrid in 2007 and helped them win two La Liga crowns before moving to Valencia last year after a season's loan at AS Roma in 2011/12.
He is a key player in Argentina coach Alejandro Sabella's World Cup team creating play for Lionel Messi and sought regular first team play, which he was not guaranteed at Valencia.
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- Velez Sarsfield