The 19-year-old midfielder penned renewed terms for three years at Camp Nou, with Barca retaining the option to extend by a further two years if Alena becomes a regular first-team fixture.
Alena scored on his senior debut against Hercules in the Copa del Rey last season and went on to make three substitute appearances in LaLiga.
A product of La Masia, having joined Barca's famed academy aged seven, Alena is viewed as one of the club's brightest prospects.
The buyout clause in his new contract is set at €75million.
Barcelona president Josep Maria Bartomeu tweeted: "The deal between @CarlesAle10 and FC Barcelona has been signed.
"Dedication and talent from the youth ranks. Congratulations!"