Capdevila who marshalled the left back position for La Roja, was a prominent member of the sides that won both the World Cup in 2010 and the European Championship in 2008.
He has played over 400 games with 36 goals in the Spanish La Liga for big guns like Espanyol, Athletico Madrid, Deportivo, and Villarreal.
The 36-year-old who played for La Liga Club Espanyol last season, said he was excited to join the club after speaking to team co-owner and Bollywood actor John Abraham.
The Kolkata franchise had announced UEFA Champions League winner Spaniard Luis Garcia as their marquee player and Antonio Lopez Habas as their marquee manager earlier this month.
"At this stage, after winning everything which is there to win, a new challenge was needed. After speaking to John Abraham about the way he is passionately looking to promote football in India and the project of NorthEast United FC, it excited me to take up the challenge.
"India is a huge country and it should be a privilege for me to be a small part in popularising this global game here and working with the young talented footballers of North East India," Capdevila stated in a release.