The singer made an estimated $80 million (£52 million) in the last 12 months, despite cancelling part of her tour to have hip surgery.
Last year she was ranked at number four, while Taylor Swift, who topped the poll in 2012, has dropped to third place with $55m (£36m).
Justin Bieber is in second place again this year with $58m (£38m).
The Forbes website claims that Lady Gaga would have earned even more had she not had to cancel part of her world tour in February.
She recently announced her next album, Artpop, would be released in November with a single due out on August 19.
To make the list, an estimation was made of the entertainment-related income of celebrities between June 2012 and June 2013 by talking to agents, managers, producers and executives.
The other artists in the top five are Calvin Harris with $46m (£30m) and Rihanna, who earned $43m (£28m).
Adele, who made the list for the first time last year, was ranked number eight making an estimated $25m (£16m).
Earlier this year the British musician won an Oscar for the James Bond theme Skyfall.
