The singers flew to Miami to apparently record a follow-up to 'The Boy Is Mine', which spent 13 weeks at number one on the Billboard Hot 100 in 1998.
"We have Both Been Blessed Beyond Measure..14 years later.. #TURNTHELIGHTSON Love @4everBrandy," Monica wrote on Twitter alongside a picture.
"I love you so much Mo thank u!" Brandy replied.
Brandy and Monica were joined by producer Rico Love, who has been working on Monica's seventh album New Life.
Brandy is also recording new material for her as-yet-untitled upcoming sixth studio album, which will mark her RCA/Chameleon Records debut.
'The Boy Is Mine' won a a Grammy for 'Best R&B Performance by a Duo or Group with Vocal' following its release nearly 14 years ago.
Watch the video for Brandy and Monica's duet 'The Boy Is Mine' below: