The Catalan giants have been chasing the skilful attacker for some time and have now finally landed their man.
The Camp Nou outfit will pay Boca Juniors £3.8million for half the player's contract, with another £3.2m due over the next two seasons to buy him outright.
Riquelme, who has always expressed a desire to play for Barca, has signed a five-year deal at Barca.
Barca president Joan Gaspart was delighted to have finally captured Riquelme's signature.
"The Argentina player is a new Blaugrana player in every effect," said Gaspart.
"We're talking about an exceptional player. We had to make sure he didn't escape."
Gaspart revealed that Riquelme could be loaned out immediately as Barcelona already have four non-European Union players, the maximum allowed under Spanish Football Federation rules.
Barca already have Brazilians Rivaldo, Geovanni and Fabio Rochemback and Argentina's Javier Saviola as their four non-EU players and unless one is sold or loaned out Riquelme will be allowed to go out on loan for first team football.
"If for technical reasons the number of four foreign players was covered, Barcelona will consider loaning the player to another club," said Gaspart.
"The player has accepted he may have to move to a series of countries for a year or six months, it is a possibility that the managers will decide in the next few weeks."
Riquelme is only Barcelona's second signing of the season, following the purchase of German goalkeeper Robert Enke from Benfica, as Louis van Gaal reshuffles the pack for next season.
