He said there is no doubt the Syrian government is responsible, and it had "crossed a red line".
He warned that intervention would lead to a proxy war with Russian Federation which would be "not only unsafe to Britain, but the entire world".
"It is about a limited and targeted strike that does not further escalate tensions in the region and that does everything possible to prevent civilian casualties", she said.
"This collective action sends a clear message that the global community will not stand by and tolerate the use of chemical weapons", May said at a press conference, calling the military action "right and legal".
The Ministry of Defence (MoD) said that four RAF Tornado jets were deployed to launch missiles at a Syrian military facility.
The Prime Minister explained: "I have taken this decision because I believe it is the right thing to do".
She added that she would address parliament on Monday.
The leader of Britain's opposition Labour Party Jeremy Corbyn said on Saturday there was no legal basis for British strikes against Syria and such action would encourage others to behave in the same way. Western governments, including Britain, blamed the government of Bashar al-Assad, a Russian ally, for the attack.
Speaking from the White House on Friday, president Donald Trump announced the military action, and said its intention was to degrade Syrian chemical weapons capabilities. Syria and its main ally Russian Federation deny a chemical attack took place.
Britain's defense ministry said "very careful scientific analysis" had been applied to maximize the destruction of stockpiled chemicals while minimizing any risk of contamination to surrounding areas.
President Trump's address regarding airstrikes on Syria
SANA reported that the joint operation was targeting military installations around Damascus and near the central city of Homs. He said the USA did not want an "indefinite presence" in Syria and would only stay until Islamic State was destroyed.
The British PM also linked the increasing use of chemical weapons with the nerve agent attack in Salisbury, insisting they were both "part of a pattern of disregard".
"I think it is right that the worldwide community has come together and said we will not accept this", she added. "We can not allow the use of chemical weapons to become normalized - within Syria, on the streets of the United Kingdom, or anywhere else in our world".
"The Syrian regime has continued to use chemical weapons and will continue to do so".
British Prime Minister Theresa May described the strike as "limited and targeted".
Mrs May said chemical weapons had "all too often" been used in recent times.
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.
Trump said strikes on Syria are under way.
He also did not rule out future military action.
"It was right to take the action that we have done in the timing that we have done", she said.