Om Prakash was arrested yesterday from his brother's house in Mirzapur district.
The watchman, who worked in the survivor's rented place in Shooltankeshwar area, is believed to have quietly entered her room on Wednesday night. When she resisted, he beat her up and snatched her mobile phone, said senior superintendent of police (SSP) Nitin Tiwari.
He fled from the scene after that.
The French national, a regular visitor to Varanasi for a decade, is undergoing treatment at a hospital where her health condition is stable.
Based on her complaint yesterday, an FIR was registered against the accused under IPC sections 376(rape) and 394 (voluntarily causing hurt in committing robbery), said Tiwari. Prakash has been arrested and sent to jail.
The French activist, who had been staying in Varanasi for a year, was sent for a medical examination. The report is awaited, police said.