"The United States knows too well the cost of terrorism directed at our own diplomats around the world, and our hearts go out to the Iranian people after this violent and unjustifiable attack," Kerry said in a statement.
The double suicide bombing killed 23 people and wounded another 146 and has been claimed by an Al-Qaeda-linked jihadist group.
"The United States strongly condemns today's senseless and despicable terrorist bombings at the Iranian Embassy in Beirut," Kerry said.
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"We extend our condolences to the victims and their families. We urge all parties to exercise calm and restraint to avoid inflaming the situation further."
Kerry said the US supports Lebanon's pledge to investigate the incident, urging that "those responsible are brought to justice."
Lebanon's government has officially declared a policy of non-involvement in the Syria conflict.
But UN political chief Jeffrey Feltman on Tuesday highlighted to the Security Council reports that Lebanese militia Hezbollah was fighting with President Bashar al-Assad's forces in Syria "in clear violation" of a statement by Lebanese parties to stay out of the war.
Iran is one of Syria's closest allies, and is the key sponsor of Hezbollah, a powerful Shiite movement that has dispatched thousands of fighters to bolster the regime in the 32-month uprising.
