It would stretch 160km (100 miles) inland from the coast, and be completed by the end of 2015, Tunisian Prime Minister Habib Essid told state TV.
The gunman who killed 38 people in a an attack on a beach resort is believed to have received training in Libya.
Tunisia declared a state of emergency following the attack last month.
The Tunisian army would build the wall, which would have surveillance centres at certain points along it, Mr Essid said.
Authorities had already tightened security following the Sousse attack, in which 30 Britons were killed, deploying more than 1,400 armed officers at hotels and beaches.
Last week, Mr Essid told the BBC that the gunman, Seifeddine Rezgui, had probably trained with the Ansar al-Sharia group in Libya, though Islamic State (IS) earlier said it was behind the attack.
In the coming weeks officials are expected to pass a counter-terrorism bill that has been in parliament since early 2014.
The Sousse attack represented the second blow in three months to Tunisia’s tourism industry, an important sector for the country.
In March, two gunmen killed 22 people at the renowned Bardo museum in Tunisia’s capital, Tunis.