The country sweetheart is estimated to have raked in $57 million in the period between May, 2011 and May, 2012, thanks to her latest hit album Speak Now, a string of sold-out tour dates and a big endorsement deal with CoverGirl cosmetics.
The 22 year old edged out 18-year-old Bieber to claim the Forbes crown - the Biebs superstar earned an impressive $55 million to land in second place.
Other stars to make the top 10 include pregnant singer Adele and Twilight couple Kristen Stewart and Robert Pattinson.
Last year, Gaga topped the Highest Paid Celebrities Under 30 poll, banking a whopping $90 million in a year.
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