Activist Jatupat "Pai Daodin" Boonpathararaksa has tested positive for the coronavirus and has been transferred to a prison hospital, a senior Corrections Department official said on Friday.
Spokesman Thawatchai Chaiwat, a deputy director-general, said the activist's Covid-19 test came back positive on Thursday.
The department had tested Mr Jatupat, human rights lawyer Anon Nampa and activist Veha Saenchonchanasuek at the Central Correctional Institute in Chatuchak district of Bangkok, Mr Thawatchai said.
The department did not say when the test samples were taken from the three anti-government protesters.
The protest leader of the Thalufah movement joins Parit "Penguin" Cheewarak and five other anti-government activists now in prison hospital for Covid-19 treatment.
Protesters rallied outside the Justice Department on Monday calling for the transfer of Mr Parit and other infected activists to Thammasat University Hospital for treatment.
A counter group of royalists submitted a petition to Justice Minister Somsak Thepsutin through a senior ministry official on Friday, opposing their transfer.
Naengnoi Assawakittikorn, a member of the defunct People's Democratic Reform Committee in Phitsanulok, said moving Mr Parit to the university hospital would create a double-standard. Other prison patients do not receive such privileged treatment, according to Mr Naengnoi.
However, the department spokesmen have said previously that prisoners do have the right to request transfer to other hospitals, and the decison would be made by doctors.