“The president has declared a state of emergency in Tunisia and will address the nation at 5 pm (1600 GMT),” Essebsi’s office said.
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.
Tunisia has faced a surge in jihadist violence since the revolution in which dozens of police and soldiers have been killed.
The June 26 beach attack was the second deadly attack on tourists in three months, following a shooting rampage at its national museum in Tunis that killed 22 people.
On Friday, Prime Minister Habib Essid acknowledged that police had taken too long to respond to last week’s attack by a gunman at the beach resort of Port El Kantaoui near Sousse.
“The time of the reaction — this is the problem,” Essid told the BBC in an interview. Police had been “blocked everywhere”, he added.
Essid spoke as Britain’s Queen Elizabeth II and Prime Minister David Cameron led a minute’s silence for the victims, 30 of whom were British.
The Islamic State group claimed responsibility for the attack by a Tunisian identified as 23-year-old Seifeddine Rezgui who pulled a Kalashnikov assault rifle from inside a beach umbrella and went on a bloody rampage at the five-star Riu Imperial Marhaba hotel.
Three Irish nationals, two Germans, one Belgian, one Portuguese and a Russian were also killed.
On Thursday, Tunisia announced it had arrested eight people, including a woman, “with direct links” to the attack.