Gaga began her career as piano-playing singer Stefani Germanotta
A songwriter and music producer who claims he helped launch Lady Gaga's career, co-writing hits and securing a record deal, is suing the US pop star.
Rob Fusari, who is credited on Gaga's album The Fame, alleges the singer denied him his share of song royalties and merchandising revenue.
On Thursday, he launched a $30.5m legal action (£20m) against the Grammy Award-winning performer in New York.
Fusari claims former girlfriend Gaga dropped him as her career took off.
Lady Gaga's spokesman, Dave Tomberlin, has yet to comment on the legal action.
In legal documents, Fusari - whose previous credits include Destiny's Child's Bootylicious - states he first came across Lady Gaga when she was performing under her real name, Stefani Germanotta, in March 2006.
Though he initially dismissed her, he realised her star potential after hearing her play the piano in his studio in Parsippany, New Jersey.
Fusari claims he spent several months "radically reshaping her approach" and transforming her into Lady Gaga - the stage name she adopted at his suggestion, he alleges.
In an interview last year, the 23-year-old pop star is quoted as saying: "I was Gaga from the time that I was 19 through my first record deal."
"I always dressed like that before people knew me as Lady Gaga. I was always that way... I stuck out like a sore thumb."
Fusari claims that he and Ms Germanotta were already romantically involved when they formed a joint business venture - Team Love Child LLC - in May 2006, to promote Gaga's career.
The songwriter also alleges he was instrumental in securing her record deal with Interscope Records, which released The Fame in 2008. The album has sold more than 3 million copies in the US alone.
With the demise of their personal relationship, Fusari claims business dealings between the pair collapsed.
The producer says he has been denied a 20% share of song royalties, 15% of merchandising revenue and other outstanding monies.
He acknowledges receiving approximately $611,000 (£403,000) but claims that is not his full share.
In January, the singer won two Grammys - best dance recording for Poker Face and best electronic/dance album for The Fame.
The Brit award-winner - who's known for hits such as Paparazzi and Just Dance, will play Birmingham, London and Manchester in May and June - after her UK tour was extended due to "overwhelming response", promoters said.