On July 16, 2018, Trump and Putin met at a conference held in Helsinki, Finland. At the conference, when asked about Russian interference in the 2016 election, Trump remarked:

I have President Putin. He just said it’s not Russia. I will say this. I don’t see any reason why it would be…So I have great confidence in my intelligence people, but I will tell you that president Putin was extremely strong and powerful in his denial today. And what he did is an incredible offer. He offered to have the people working on the case come and work with their investigators, with respect to the 12 people. I think that’s an incredible offer.

Before the conference, Trump and Putin had a two-hour meeting in private, with only a single translator present.

After the conference, Trump reiterated his position on Twitter.

US intelligence agencies were in agreement that Russia had actively interfered in the 2016 election. In April, 2020, the Republican-controlled US Senate affirmed their findings.

ABC News – Trump casts doubt on US intelligence, calls Putin’s meddling denial ‘strong and powerful’
MarketWatch – Trump Today: President says Putin denial of election interference was ‘powerful’ 
Business Insider – Meet Marina Gross, the interpreter who was the only other American in the room for Trump and Putin’s 1-on-1 meeting in Helsinki
Vox – Read the full transcript of the Helsinki press conference
Wikipedia – 2018 Russia–United States summit
Politico – Senate Intel report confirms Russia aimed to help Trump in 2016