He was the world’s oldest sitting president. He was admitted to hospital on Wednesday but officials did not say why he was receiving treatment.
Mr Essebsi won Tunisia’s first free elections in 2014 following Arab uprisings across the region.
He was admitted to hospital last month after suffering what officials said was a severe health crisis.
They gave no further details at the time. But Prime Minister Youssef Chahed, who visited him in hospital, urged people to stop spreading “fake news” about his condition.
According to the country’s constitution, Mr Chahed can take over as president for no more than 60 days or until a replacement is elected.
Earlier this year, Mr Essebsi announced that he would not stand in elections expected in November.
He told a meeting of his ruling Nidaa Tounes party that someone younger should take charge. He said it was time to “open the door to the youth”.
Mr Essebsi was a former lawyer who studied and trained in the French capital Paris. During his long political career he also served as interior minister and Speaker of parliament.
Former Tunisian President Zine el-Abedine Ben Ali was ousted in 2011 after 23 years in office.
Since then, Tunisia has won praise as the only democracy to emerge from the revolutions of the so-called Arab spring.
But in recent years the country has suffered attacks by Islamists and economic problems, with unemployment a persistent issue.