The attacks early on Saturday come in response to a suspected chemical weapons attack in the former rebel stronghold of Douma last weekend.
“I ordered the United States armed forces to launch precision strikes on targets associated with the chemical weapon capabilities of Syrian dictator Bashar al-Assad,” Trump said from the White House late on Friday.
Explosions were reportedly heard in Damascus, Homs and elsewhere in Syria, as Trump spoke.
A US official told Reuters news agency the attacks were aimed at multiple targets and involved Tomahawk cruise missiles.
UK Prime Minister Theresa May confirmed Britain’s involvement in the intervention, saying: “We would have preferred an alternative path. But on this occasion there is none.”
She said the attacks were not about “regime change” or “intervening in a civil war”, but were to “deter the use of chemical weapons” by the Syrian government.
French President Emmanuel Macron also confirmed France’s involvement in the attack.
The purpose of the campaign is to “establish a strong deterrent against the production, spread and use of chemical weapons,” Trump said.