He was admitted to the All India Institute of Medical Sciences in New Delhi on Wednesday evening.
The 89-year-old Congress leader was admitted for evaluation of fever and his condition is stable, AIIMS officials said.
Earlier this year, the Congress veteran and Rajya Sabha member had been admitted to the hospital after he tested positive for COVID-19 during the second wave of infections.
Pranav Jha, Congress secretary in-charge of communications, said Singh is "undergoing routine treatment.” He also dismissed "unsubstantiated rumours" regarding his health.
Soon after news of Singh's hospitalisation came, political leaders cutting across party lines took to Twitter to wish him a quick recovery.
"I pray for the good health and speedy recovery of Dr Manmohan Singh Ji," Prime Minister Narendra Modi tweeted on Thursday.
Health Minister Mansukh Mandaviya also met Singh and enquired about his health at the hospital. In a tweet in Hindi, Mandaviya said he wished Singh a speedy recovery.