The US magazine has estimated that the rapper's wealth now "conservatively totals" $1bn (£800m).
Forbes says the husband of singer Beyonce has succeeded because he built brands rather than just endorsed them.
In its rankings, Forbes rejected claims that rapper and producer Dr Dre had reached billionaire status.
Jay-Z, born Shawn Carter, grew in one of New York City's most notorious areas.
He hit fame in 1996 with his debut album Reasonable Doubt. His 2001 album The Blueprint was in March added to the Library of Congress' National Recording Registry because it was deemed "culturally, historically, or aesthetically significant".
Forbes said it had estimated Jay-Z's wealth by adding various assets and then "subtracting a healthy amount to account for a superstar lifestyle".
Among the 49-year-old's assets are:
A $75m music catalogue
Stakes in Armand de Brignac champagne and D'Usse cognac, worth $410m in total
A stake in Uber worth about $70m
Property in fashionable parts of New York and Los Angeles
A $100m stake in the music streaming service Tidal
An art collection valued about $50m
His superstar wife is reportedly worth about $335m, made mostly from music and endorsements, and the couple have had a joint net worth over $1bn for several years.