Reverend Watareka Vijitha has been arrested after he was discharged from a hospital here, police said.
Vijitha, a critic of hardline Buddhist group Bodu Bala Sena (BBS), was found with injuries which the police said had been self inflicted.
The monk had initially claimed that he was abducted by men in robes and later he found himself in hospital.
The BBS had accused the monk of siding with the Muslim minority.
The BBS activists were blamed last week for the anti-Muslim riots in the southwestern coastal area of Aluthgama which killed four people and injured nearly 100.