The ex- Ajax man has been deemed surplus to requirements at San Siro, and it was expected that he will leave Jose Mourinho's team during the current transfer window.
"FC Barcelona and Inter Milan have reached the beginning of an agreement for the transfer of the Brazilian Maxwell," Barcelona announced on their official website.
"The deal shall go through for a fee of 4.5 million Euros with an additional 0.5m Euros in variable payments."
Maxwell has been Inter's first choice full-back since he joined the Nerazzuri in 2006.
However, the emergence of Italy youngster Davide Santon changed Mourinho's plans regarding Maxwell's future at the club.
The 28-year-old is set to be a back up replacement at Barca after the Blaugrana parted ways with veteran Brazilian full-back Sylvinho.