The man was arrested on Wednesday in connection with three sex attacks in the Brixton Hill and Clapham areas on Monday.
He is being held in custody at a south London police station, Scotland Yard said.
The arrest comes after a video shared on social media showed a woman being attacked as she fumbled for her keys on a doorstep.
She was approached from behind by a man who allegedly put his hands up her skirt as he dragged her to the ground. He can be seen fleeing the scene after a brief struggle.
The victim told Mail Online: “I was so afraid. That's all I could think of.
"That's all that was going through my head.
“It was 8pm at night, there were people walking home and it was busy. I have lived on that road for more than five years and I have never felt unsafe there before."