Prime Minister Nawaf Salam signed a decree announcing the measure, according to official reports.
At least 254 people have been killed in the strikes, authorities say, with the toll expected to rise as rescue operations continue.
Under the decree, state institutions, public organisations and municipalities will be closed, while flags will be flown at half-mast nationwide.
Broadcast schedules on radio and television are also set to be adjusted in line with the mourning period.
The announcement comes amid growing concern over the humanitarian impact of the attacks and the broader escalation in the region.
