The 20-year-old former Arsenal youth player joined Brighton in January from Preston, where he made 49 appearances.
He has yet to play for Albion and was loaned to Crawley for the final months of last season.
Dons manager Derek McInnes told the club's official website: "Jeffrey is a player we have been aware of for some time and will offer additional competition in the wide areas, where he brings real pace."
Monakana joins Shaleum Logan, Ashton Taylor, David Goodwillie and Scott Brown signing for the Dons this summer. He is not eligible for Thursday's Europa League qualifier against Groningen.