“The bodies of 23 people have been found during an on-site inspection,” the Investigative Committee said in a statement on Sunday, adding that a further 23 people were hospitalised.
Seventy patients and four nurses were in the ward when the fire broke out shortly before midnight Saturday at the hospital in the village of Alferovka, located in the southern Voronezh region.
Most of those who perished in the blaze were in their 60s and 70s, although some were in their 40s and 50s, according to a list of patients released by the emergencies ministry.
The state-owned Rossiya 24 rolling news channel said those who died were bedridden patients who had been given tranquillizers.
“They simply did not wake up,” a correspondent reported from the scene.
The fire reduced the wooden hospital building to its scorched foundations, with footage showing rescue workers combing through the smoking ruins.
It took more than 440 firefighters and emergency workers around three hours to bring the fire under control.
The Investigative Committee, which reports directly to President Vladimir Putin, said it opened a criminal probe into the deaths on suspicion of negligence.
Experts are looking into what triggered the blaze, investigators said.
The fire was the latest tragedy to hit a psychiatric institution in Russia, where outdated Soviet-era infrastructure is still in widespread use and managers often take a lax approach to fire safety.
Scores of people also die in house fires each year.
A fire at a psychiatric hospital in northwest Russia in September 2013 left 37 people dead, while a blaze in a psychiatric ward near Moscow in April of the same year killed 38.
In 2009, 156 people were killed in a nightclub fire in the city of Perm, some 1,200 kilometres (700 miles) east of Moscow in one of the deadliest accidents in Russia’s modern history.