This is the first direct US assault on Assad's forces in six years of war.
The US struck a military base of the Syrian Air Force.
A US Navy destroyer fired 59 cruise missiles from a warship off the coast of Lebanon.
The strikes destroyed 12 fighter jets, putting the air base "completely out of action," a military source said.
Six Syrian soldiers were killed. Russia was notified before the assault.
The strike was in retaliation to a chemical attack in Idlib.
The order came directly from President Donald Trump less than 72 hours after a chemical attack in Khan Shaykhun in Syria's Idlib province.
Tuesday's attack killed at least 80 people and injured hundreds of others.
Trump said that the graphic images of the aftermath "had a big impact" on his view toward Syrian regime leader Bashar al Assad.
"Years of previous attempts at changing Assad's behaviour have all failed, and failed very dramatically," Trump said, referring to Syrian regime leader Bashar al Assad.
Doctors have confirmed that the chemical agent sarin was used.
Autopsies conducted on victims of the attack who died after they were rushed to a Turkish hospital just over the border confirmed that they died of the effects of sarin – a highly lethal nerve agent that can kill a person within minutes of contact.
Pentagon officials are highly confident Assad is responsible.
They said intelligence showed that the plane that struck Khan Shaykhun was a Syrian Sukhoi-22 fighter jet deployed from Shayrat air base, and that the facility was used to store its chemical stockpiles.
This figure shows the flight path of the Syrian aircraft that carried out the Khan Sheykhoun chemical attack, the Pentagon claims. pic.twitter.com/JXUoswsRNj— Christiaan Triebert (@trbrtc) April 7, 2017
Recent UN documents show that Assad had not completely disposed of chemical weapons stockpiles.
Documents published in January 2017 by the Organisation for the Prohibition of Chemical Weapons, a United Nations-mandated body, showed that three chemical weapons production facilities in Syria were still operational and had not been dismantled.
The Syrian regime is denying responsibility.
Regime officials have continued to deny responsibility for the chemical attack in Khan Shaykhun, instead offering the explanation that the toxic nerve agents belonged to rebel fighters and were being stored in a warehouse.
A spokesperson for the Russian presidency said the US strikes came "under a false pretext."
The Turkish Foreign Ministry issued a statement in support of the US attack, saying: "Any step which clears the way for a political solution in Syria will be supported by Turkey."