The 28-year-old defender was a member of the tournament-winning franchise Chennaiyin FC last year, starting in 12 matches for them.
NorthEast United's head coach, Sergio Farias expressed his approval over the signing in a release: "This season we are picking players based on their suitability to fit into our philosophy. Mailson is someone who can give us that edge. His experience in the ISL is also a huge factor."
Alves, upbeat about the upcoming season and happy to sign for the team, said: "I am looking forward to working with the head coach and joining NEUFC. We are putting together a special team this season and I am raring to give my best for the fantastic fans and this unique club."
NEUFC's director of football, Ardeshir Jeejeebhoy said: "In Mailson, we have someone who has the experience of playing in Indian conditions and carries the achievement of being an ISL winner. Sergio wanted us to get him on board and I am pleased that we could make that happen. We hope that he will bring his winning mentality to the table and help the team challenge for honours."
NorthEast have earlier signed Japanese midfielder Katsumi Yusa and Indians Subrata Paul, Rowllin Borges, Sumit Passi and Nirmal Chettri, among others.