After face-to-face talks with Russian President Vladimir Putin, Mr Trump contradicted US intelligence agencies and said there had been no reason for Russia to meddle in the vote.
Mr Putin reiterated that Russia had never interfered in US affairs.
The two men held nearly two hours of closed-door talks in the Finnish capital Helsinki on Monday.
At a news conference after the summit, President Trump was asked if he believed his own intelligence agencies or the Russian president when it came to the allegations of meddling in the elections.
"President Putin says it's not Russia. I don't see any reason why it should be," he replied.
US intelligence agencies concluded in 2016 that Russia was behind an effort to tip the scale of the US election against Hillary Clinton, with a state-authorised campaign of cyber attacks and fake news stories planted on social media.
What has US reaction been?
Senior Republican Senator Lindsey Graham, a member of the Senate Armed Services Committee, said Mr Trump had sent the Kremlin a message of US "weakness".
He tweeted: "Missed opportunity by President Trump to firmly hold Russia accountable for 2016 meddling and deliver a strong warning regarding future elections."
Fellow Republican Senator Jeff Flake - a staunch critic of President Trump - called his words "shameful".
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