The prime minister was asked if she was “alarmed” by the failure to raise the attack - blamed on Russian military intelligence service – at the weekend talks between the two presidents.
Speaking in the Commons, Ms May confirmed she had asked Mr Trump to confront Mr Putin, when they spoke on his controversial trip to the UK just two days earlier.
But, instead, she pointed to his earlier response to the Salisbury attack, telling MPs: “The United States stood alongside us, as did many other nations across the world, and took action against Russia.”
Earlier, Ms May provoked howls of laughter when she argued she had been “agreeing the future of NATO with President Trump”, in their talks.
Mr Trump was briefed on the attempted assassination of Russian former double agent Sergei Skripal and his daughter Yulia in Salisbury in March.
The same batch of Novichok then killed Dawn Sturgess and left her partner Charlie Rowley critically ill last month.
Ahead of the Trump-Putin summit in Helsinki, Miss Sturgess's son Ewan said: “I would like him to raise mum's case with the Russian president. We need to get justice for my mum.”
At prime minister’s questions, Keith Simpson, a Tory backbencher, said Mr Trump had “acted in a very bizarre way over intelligence” in Helsinki.
“Is she not alarmed at the way he refused to challenge President Putin over the Russian activity which resulted, recently, in the death of a young women here in Salisbury?” he asked.
Ms May replied: “I did raise the incident in Salisbury, what happened in Salisbury and the fact that we have seen someone in the UK here die as result of contact with a nerve agent.
But, ducking what was said in Helsinki, she pointed to Mr Trump’s expulsion of 60 Russian diplomats in the immediate aftermath of the earlier poisoning.
“That gave the very clear message that we will not accept this behaviour. This is not behaviour that Russia can conduct with impunity and we will continue to act together,” the prime minister told MPs.
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