The 21-year-old has become the Gunners' second arrival of the summer window following Nuno Tavares's capture from Benfica and leaves Anderlecht after 11 years at the club.
Lokonga - who will wear the number 23 shirt - is understood to have penned a five-year deal with Arsenal until the summer of 2026, and Anderlecht will reportedly receive €21m (£18m) for his sale.
Speaking to the official Arsenal website, manager Mikel Arteta said: "Albert is a very intelligent player who has shown great maturity in his performances during his development. He has been coached well by Vincent Kompany and his team at Anderlecht.
"I know Vincent very well and he talks so highly about Albert and the positive impact he had to Anderlecht in recent seasons. We're confident Albert's ready for the next stage in his development and we're looking forward to welcoming a new member to our squad with his quality and presence."
Technical director Edu added: "Albert is a player we've been tracking for some time, and he will bring us strength in depth in the centre of midfield. We look forward to his continued development with us."
Lokonga's arrival comes at a time where Granit Xhaka is expected to complete a move to Roma, and the Belgium Under-21s international leaves Anderlecht after scoring three goals and assisting seven more in 78 appearances across all competitions. body check tags ::