The landmark number is a first for both the singer and Twitter itself.
The 25-year-old singer, who was in London on Saturday, celebrated the milestone by tweeting, "10MillionMonsters! I'm speechless, we did it! Its an illness how I love you. Leaving London smiling."
The 'Paparazzi' hitmaker was also the first person to hit 10 million fans on Facebook, as well as 1 billion views on YouTube.
Gaga recently partnered with social gaming company Zynga to launch GagaVille, a Gaga-themed offshoot of Zynga's hit game, FarmVille.
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