The deal is worth 14 million euros (£12m) and is subject to the 23-year-old left back, a Barca youth academy graduate who is representing Spain at Euro 2012, passing a medical, the club said on their official Twitter feed.
Barca were in the market for a left-sided full-back after Abidal had a liver transplant earlier this year and it remains uncertain whether the France international will be able to make a return to action.
After some impressive displays for Valencia, the pacy Alba made his debut for Spain against Scotland in Alicante in October and has since made the left-back position vacated by Joan Capdevila his own earning 10 caps.
Born in the same Catalan town as Barca keeper Victor Valdes, he started out in the club's youth ranks but was released and joined regional league side UE Cornella before Valencia snapped him up at the age of 17 for a bargain 6,000 euros.
Roving down the wing, Alba uses his explosive pace to surge past his markers and deliver inviting crosses and he has been a big threat to Spain's opponents at the finals in Poland and Ukraine as they bid to retain their continental title.
He helped serve up one of the moments of the tournament in the 2-0 quarter-final win against France when he burst into space and sent over a perfect cross for Xabi Alonso to head home Spain's opening goal.
Alba has established himself as Spain's top left-back after an impressive season at Valencia and some solid international appearances. Pacy and tenacious in the tackle, he is adept at getting forward to send over dangerous crosses.