The Prime Minister added: “We have done it because we believe it was the right thing to do.”
Mrs May faced a grilling by MPs in the Commons over a series of missile strikes launched on Saturday.
She insisted the Syrian airstrike on the Assad regime was in “Britain’s best interest” after a chemical weapons attack in Douma, which is thought to have killed around 75 people.
It came as a row erupted in Commons as opinion was divided over Mrs May’s decision.
The PM’s decision to launch airstrikes without parliamentary approval has led to criticism from MPs.
But she defended her decision not to recall Parliament, suggesting the “security” of the operation could have been compromised.
“The speed with which we acted was essential in co-operating with our partners to alleviate further humanitarian suffering and to maintain the vital security of our operations,” she said.
The decision required the evaluation of intelligence “much of which was of a nature that could not be shared with Parliament”.
It came as opinion in the Commons was divided over the action, with Labour backbenchers praising Mrs May for her decision.
Senior Tory Ken Clarke called on Mrs May to establish a cross-party commission to look at Parliament’s role in approving military action.
While Labour leader Jeremy Corbyn repeated his assertion that the military action was “legally questionable”.
But there were cries of “shame” from the Tory benches as he told Mrs May she “is accountable to this Parliament, not to the whims of the US President”.
Speaker John Bercow had to intervene to calm MPs, telling them Mr Corbyn must be afforded the same “respectful quiet” given to Mrs May.
Mr Corbyn insisted Attorney General Jeremy Wright’s legal advice must be published in full on Monday.
He also raised concerns over the use of banned cluster bombs and white phosphorous by Saudi Arabia as he raised humanitarian concerns over Yemen.
Mr Corbyn asked: “Will the Prime Minister commit today to ending support to the Saudi bombing campaign and arms sales to Saudi Arabia?”
The Labour leader later cited the Chilcot Inquiry into the Iraq War and asked the Prime Minister whether she agreed with its “key recommendation” that there needed to be greater checks on intelligence when it was used to make the case for Government policies.
He said: “Given that neither the UN nor the OPCW (Organisation for the Prohibition of Chemical Weapons) has yet investigated the Douma attack, it is clear that diplomatic and non-military means have not been fully exhausted.
“While much suspicion rightly points to the Assad government, chemical weapons have been used by other groups in the conflict.”
Mr Corbyn added that it was “vitally important” that OPCW inspectors were allowed to investigate and report their findings.
Meanwhile, Labour backbenchers were cheered as they praised Theresa May for taking military action in Syria.
Former Labour ministers lined up to thank Mrs May for taking the “absolutely right” course of action, with some warning of the dangers of inaction in what appeared to be implicit criticism of the party’s front bench.
Chris Leslie, who served as a minister under Tony Blair, said: “Pinpointing and degrading (Bashar) Assad’s chemical weapons was necessary and appropriate, and intervening to save civilians from future gas attacks – while not without risk – was absolutely the right thing to do.
“Would the Prime Minister also agree that a policy of inaction also would have severe consequences, and that those who would turn a blind eye, who would do nothing in pursuit of some moral high ground, should also be held accountable – for once – today as well?”
Ian Blackford, the SNP’s leader in Westminster, said it was “perfectly possible” for the House to have been recalled before Saturday’s air strikes.
Liberal Democrat leader Sir Vince Cable said he regretted the decision not to recall Parliament for a vote.
However, Tory former defence secretary Sir Michael Fallon said: “Would the Prime Minister accept that the public well understand that when our forces need to act quickly and decisively and safely in concert with our allies, it must be right to authorise strikes without giving notice?”
Sir Vince also asked the Prime Minister if she would order fresh strikes if the Syrian regime used chemical weapons again, to which she replied: “Nobody should be in any doubt of our resolve to ensure that we cannot see a situation where the use of chemical weapons is normalised.”
DUP Westminster leader Nigel Dodds praised Mrs May’s statement as it addressed the “challenges of these difficult times”, adding this was in “stark contrast” to Mr Corbyn’s contribution.
He said the PM was “utterly justified” in the action she took, also telling the Commons: “She should have the support of every right-thinking member of this House in upholding international law and defending the national interest of the United Kingdom.”