In a tweet on Tuesday, Zarif wrote, "Our thoughts and prayers are with the great and resilient people of Lebanon."
"As always, Iran is fully prepared to render assistance in any way necessary," he added.
"Stay strong, Lebanon," Zarif said.
A massive explosion rocked Beirut, Lebanon, with the force of an earthquake followed by a shock wave that devastated much of the city and is believed to caused hundreds of casualties.
The blast appeared to have followed a fire that had broken out in the city's port area, based on a video from the scene. The cause of the blast was immediately unknown. The force of the blast shook buildings, which were then hit again by the shock wave that blew out windows, sending shards of glass flying.
Lebanese Red Cross official Georges Kettaneh said there were dead and wounded, but did not have an exact figure, just saying there were hundreds dead or wounded.
Beirut's governor Marwan Abboud called it a "national catastrophe" and the prime minister declared a day of mourning.