A statement from the president’s office said Essebsi will give a speech on national TV at 4pm GMT, when he will address the nation, according to Reuters.
The declaration follows last week’s attack in which a lone gunman entered a resort at the Mediterranean coast town of Sousse and began systematically killing tourists, majority British, but also some from Germany and Belgium.
Those killed included 30 Britons.
Lorna Carty from Robinstown, Co Meath, and Martina and Laurence Hayes from Westlodge, Athlone, were killed last week when Tunisian student Seifeddine Rezgui opened fire on a beach in the Sousse region.
On Friday, the authorities admitted for the first time that there had been security failures, with Prime Minister Habib Essid saying: “The time of the reaction – this is the problem”.
He said Rezgui had probably trained with the Ansar al-Sharia group, though Islamic State (IS) earlier said it was behind the attack.
Eight people have been arrested in connection with the attack. Immediately after the Sousse attack, the prime minister pledged to post armed guards at tourist sites and close mosques outside government control.
Analysts say Tunisia has been put at risk by the chaotic situation in neighbouring Libya, and by the threat posed by Tunisians who have gone to fight in Syria and Iraq returning home.
An earlier state of emergency, granting special powers to the police and army, was lifted in March 2014, having been in force since longtime president Zine El Abidine Ben Ali was ousted in a 2011 revolution.
Islamic State (Isis) claimed responsibility for the massacre, which was the most deadly in Tunisia’s recent history.
In March, gunmen killed 22 people, again mostly tourists, at The National Bardo Museum outside Tunis.