Reis was reported to be close to joining Barca by his agent earlier this month and Groningen announced that the 18 year-old has agreed a contract running until 2022, with the option of two further years.
The multi-season deal contains a €100m release clause.
Reis first came to prominence in October 2017, when his goal at Sparta Rotterdam made him the first player born in the 21st century to score in the Eredivisie.
The Netherlands Under-19 international turns 19 next month and, having played 50 senior games for Groningen, he is expected to start life in Catalonia as part of the Barca B squad.
"It's a dream come true for me, I'm very happy." Reis told Barca TV. "I still do not believe it, I dreamed every night of this moment when I played football with my friends.
"I want to learn so much from the rest of the players, who are the best in the world."
He explained that one of those players in particular serves as an inspiration.
"My position is a defensive midfielder, but I can play also offensive a little bit. I can play the two positions," Reis added.
"My goal is to play with the first team and learn from Sergio Busquets, [and] so many players here."
Barcelona has completed three signings for next season, with Atletico Mineiro's Emerson and Ajax's Frenkie de Jong – a Dutch midfielder arriving at considerably greater expense – having already agreed terms.