More than 22,000 people have tested positive inside jails, where inmates living cheek by jowl have been encouraged to keep wearing their masks even while they sleep.
Authorities have floated plans to give early releases to prisoners with underlying medical conditions and have announced funding for more testing and medical care in recent days.
But those behind bars say they have been kept in the dark about the seriousness of the outbreak.
â€œPrisoners donâ€™t have the knowledge to protect themselves,â€ said Somyot Prueksakasemsuk, a high-profile activist facing charges under Thailandâ€™s harsh royal defamation law.
Somyot was bailed last month and told AFP that he had not been tested for Covid-19 once during his 10-week stint in custody.
He was not worried about contracting the disease while in jail because he had no idea about the level of risk.
â€œBut after this Iâ€™m so scared [for everyone still inside] … if you are inside the prison you are at risk, itâ€™s unavoidable,â€ he said.
Thailandâ€™s prison outbreak has skyrocketed from just 10 publicly announced cases a month ago and sparked growing public concern after a handful of prominent activists contracted the illness.
Among them was student leader Panusaya â€œRungâ€ Sithijirawattanakul, who helped lead a series of rallies last year demanding political reforms in the kingdom, and who tested positive after she was released on bail.
The Thai prison population stood at around 311,000 earlier this year, the International Federation for Human Rights said â€“ more than two and a half times the systemâ€™s official capacity.
Four inmates in every five are serving time for drug charges because of harsh anti-narcotics laws that can see offenders jailed for a decade for possessing just a few methamphetamine pills.
Many cells are so packed with bodies that some inmates only have half a metre (less than 2ft) of space.
â€œThat is less room for a body than the inside of a coffin,â€ justice minister Somsak Thepsutin told local media in February.
Officials have tested more than 36,000 inmates in recent weeks and begun delivering Covid vaccines to inmates and prison staff.
Somsak said he was examining ways to give early releases to prisoners with underlying medical conditions, possibly through a royal pardon.
Even if the plan goes ahead, prisoners will still have to complete a quarantine before returning home.