By Agence France-Presse
"This collective action sends a clear message that the international community will not stand by and tolerate the use of chemical weapons," May told a press conference.
She said that at an emergency cabinet meeting in London on Thursday "we agreed that it was both right and legal to take military action" after hearing legal advice.
"I believe that the action taken will have significantly degraded the Syrian regime's ability to use chemical weapons," she said.
"While the full assessment of the strike is ongoing, we are confident of its success," she added.
Four British fighter jets struck a military base near Homs where Britain said Syrian government forces were holding chemical weapon components.
"We have hit a specific and limited set of targets," she said.
Asked why she had proceeded without consulting parliament, May cited operational security consideration.
"It was right to take the action that we have done in the timing that we have done," she said.
May said she would address parliament on Monday about the strikes.