The 27-year-old singer came first in the annual ranking - which is determined by earnings, social media influence and press mentions - after banking $80 million (£109 million) between June 2012 and June 2013.
She amassed the huge figure largely thanks to her 'Born This Way' world tour grossing a massive, despite it being cut short due to a hip injury.
The eccentric performer - who also ranked in second place on Forbes' Most Powerful Celebrities list - beat Justin Bieber into second place.
The 19-year-old pop star made an impressive $58 million (£37 million) from his record sales, sell-out tour and business interests in companies such as music-streaming website Spotify, Enflick and Tinychat.
Taylor Swift with earnings of $55 million (£36 million), superstar DJ Calvin Harris, who banked $46 million (£30 million), and Rihanna, who filled her account with $43 million (£28 million), rounded off the top five.
Other stars to make the list included 'Hunger Games' star Jennifer Lawrence - who made $26 million (£17 million) - Adele, and Taylor Lautner also were included in the top 10.
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Forbes Magazine's Top Earning Celebrities Under 30, top 10:
1. Lady Gaga ($80m / £109m)
2. Justin Bieber ($58m / £37m)
3. Taylor Swift ($55m / £36m)
4. Calvin Harris ($46m / £30m)
5. Rihanna ($43m / £28m)
6. Katy Perry ($39m / £25m)
7. Jennifer Lawrence ($26m / £17m)
8. Adele ($25 / £16m)
9. Kristen Stewart ($22m / £14m)
10. Taylor Lautner ($22m / £14m)