The Spanish Primera Liga club and the Argentine champions "have reached an agreement for the release of the player Fernando Gago until the end of this season," said a statement on Valencia's website.
Gago, who is unsettled at Valencia and appeared earlier this month to be on his way back to his first club Boca Juniors, will strengthen coach Ricardo Gareca's squad as Velez bid for a second Libertadores Cup this year.
Velez won the Inicial Tournament, first of two championships in the Argentine season, in December, their third title under Gareca since 2009. The Final Tournament starts on February 8 with Velez away to Argentinos Juniors.
This month Boca secured the signing of former Velez forward Juan Manuel Martinez from club world champions Corinthians of Brazil, the biggest coup up to then.
However, Gago's greater experience and importance to Alejandro Sabella's Argentina team as a playmaker make his move the biggest of the summer recess.
Velez general manager Cristian Bassedas, a former Velez, Newcastle United and Argentina midfielder, said: "His joining us is going to help us a lot.
"It will do him a lot of good to come back to Argentina and above all play regularly," Bassedas was quoted as telling the Argentine radio station La Red.
"He's coming to Velez because the club is in exceptional shape. [The team] are the last champions, and it's a model club," he added of what is widely regarded as the best run football club in Argentina.
"The chance [of a move] came last week when Fernando had an argument at Valencia and was separated from the first team squad. He found he was not going to be considered by the coach and decided to look for a club."
Gago, 26, joined Real Madrid from Boca in 2007. He spent the 2011/12 season on loan at AS Roma before joining Valencia.
Nine times Argentine champions Velez won the Libertadores Cup, South America's equivalent of the European Champions League, in 1994. The group stage kicks off on February 12.
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