The US President re-tweeted three posts by Jayda Fransen on Wednesday.
The posts included unverified videos titled “Muslim Destroys a Statue of Virgin Mary” and “Muslim migrant beats up Dutch boy on crutches”.
Fransen, 31, from Penge, south-east London, is on bail facing four charges of causing religiously aggravated harassment as part of a Kent Police investigation into the distribution of leaflets and the posting of online videos during a trial held at Canterbury Crown Court in May.
She will also appear in court in Northern Ireland in December charged with using threatening and abusive language in connection with a speech she made at an anti-terrorism demonstration in Belfast on 6 August.
Mr Trump, who is known for his unguarded messages on social media, has 43.6 million followers on Twitter.
In June this year, an analysis found Facebook and Twitter had failed to take down a number of misleading videos posted by Britain First despite them having received hundreds of thousands of views.
Videos posted by the nationalist group often target minority groups, and analysis found they consistently posted false claims about the circumstances being shown - including untrue claims that Muslims and migrants had attacked women and police.
The re-tweets were met with criticism on Twitter.
Brendan Cox, widower of MP Jo Cox who was murdered by a right-wing extremist, said: “Trump has legitimised the far right in his own country, now he’s trying to do it in ours. Spreading hatred has consequences & the President should be ashamed of himself.”
Paul Joseph Watson, the UK-based editor of far-right conspiracy website Infowars, said: “Yeah, someone might want to tell whoever is running Trump’s Twitter account this morning that re-tweeting Britain First is not great optics.”
A tweet from Fransen’s account, which is verified by Twitter, appeared to celebrate the re-tweets.
She said: “The president of the United States, Donald Trump, has re-tweeted three of deputy leader Jayda Fransen’s Twitter videos. Donald Trump himself has re-tweeted these videos and has around 44 million followers. God bless you Trump. God bless America.”
The first video, which Fransen claimed shows a Muslim migrant attacking a Dutch man on crutches, was also shared by one of Mr Trump’s most vocal supporters, conservative commentator Ann Coulter.
The other two videos appear in Fransen’s timeline but not in sequence.