She said it was clear that only the Assad regime could have conducted the suspected chlorine attack on Douma that killed dozens.
French President Emmanuel Macron said the strikes had been limited so far to Syria's chemical weapons facilities.
The Green Party, which has one MP, said May had "trampled over parliamentary democracy" and demanded a vote in the House of Commons on the strikes.
"It was right to take the action that we have done in the timing that we have done", she said.
The Labour leader added that weapons inspectors were on their way to verify the use of chemical weapons by the Assad regime.
"We will not sacrifice the worldwide order that we have collectively to the Russian desire to protect its ally at all costs".
On this target, 76 missiles - 57 U.S. Tomahawk Land Attack Missiles (TLAMs) and 19 joint air-to-surface missiles - were used in the strikes.
May doesn't have a majority in Parliament, but the Democratic Unionist Party, which is propping up her government, said it backed her actions.
"Last Saturday the Assad regime again deployed chemical weapons to slaughter innocent civilians, this time in the town of Douma near the Syrian capital of Damascus".
The UK Cabinet agreed Thursday "that the Assad regime has a track record of the use of chemical weapons and it is highly likely that the regime is responsible for Saturday's attack", according to the Downing Street statement. We can not allow the use of chemical weapons to become normalised - within Syria, on the streets of the United Kingdom, or anywhere else in our world.
Mrs May said Britain and its allies had sought to use every diplomatic means to stop the use of chemical weapons, but had been repeatedly thwarted, citing a Russian veto of an independent investigation into the Douma attack at the UN Security Council this week. The United States responded with 58 missile strikes that destroyed 20% of the Syrian airforce.
Germany will not join any military strikes, but supports Western efforts to show chemical weapons use is unacceptable, Chancellor Angela Merkel says.
The three nations have been working on a plan for military strikes in response to last week's attack in Douma.
The MoD added the facility was located "some distance" from "concentrations of civilian habitation", and the risk of contamination to the surrounding area had been minimized.
"Initial indications are that the precision of the Storm Shadow weapons and meticulous target planning have resulted in a successful attack", it said.
The one-off missile strike in April 2017 targeted the airfield from which the Syrian aircraft had launched their gas attack.
May is not obliged to win parliament's approval before ordering military action, but a non-binding constitutional convention to do so has been established since a 2003 vote on joining the USA -led invasion of Iraq.
"No decision as yet, the cabinet is meeting in full at 1530 to discuss", Brexit minister David Davis said.
Formally, the prime minister has the right to go to war without approval from parliament, but a convention has been established in previous conflicts where MPs have a vote either before or shortly after military action begins.
At least six loud explosions were heard in Damascus, and smoke was seen rising over the city even while Trump delivered his remarks.
British opposition lawmakers are calling for Parliament to be given a vote before any military action.
Or it was the precursor to wider military action.