Entering and exiting the Red Sea city of Jeddah will be banned as of Sunday, the Saudi state news agency SPA reported, citing an unnamed official at the Interior Ministry.
A nationwide night-time curfew imposed earlier this week will be extended by four hours in Jeddah as of Sunday, according to the official.
The curfew now begins in the city each day at 3 pm (1200 GMT) and runs through until 6 am.
On Thursday, the Saudi authorities enforced a total lockdown on the capital Riyadh, along with Mecca and Medina, Islam's two holiest cities, as part of tighter restrictions against the virus.
The kingdom, the birthplace of Islam, had already halted religious journeys to Mecca and Medina.
It has also renewed a halt to international and domestic flights, as well as local transport until further notice, SPA reported.
Saudi Arabia has reported four deaths from Covid-19, the respiratory disease caused by the virus, among a total of 1,203 infection cases, the highest number in an Arab country.