The announcement comes after the two, together since fall 2008, attended Thursday night's Black Ball in London, which benefits Keys's Keep a Child Alive charity.
"They're very happy," says a friend of the couple, while reps confirm to PEOPLE the nuptials will take place "in a private ceremony later this year."
Rapper Swizz Beatz (real name: Kasseem Dean), 31, has produced music for Keys, 29, and worked with artists including Jay-Z, Beyoncé, Jennifer Lopez and Gwen Stefani.
It will be the first marriage for Grammy-winner Keys – and their first child together. Swizz Beatz is father to Kasseem Jr., 3, and son Prince Nasir, 9, both from previous relationships.