Ben Carson takes a narrow lead in in a New York Times/CBS News poll, ousting the New York businessman who has led the pack for nearly four months.
The news comes on the eve of the third TV debate for Republican contenders.
The next-ranked candidates are Marco Rubio, Jeb Bush and Carly Fiorina but they are a long way behind.
The conservative Mr Carson, who has expressed extreme views on a range of issues, has 26% of Republican primary voter support, according to the telephone poll of 575 voters.
His lead of 4% over Mr Trump is well within the 6% margin of error.
Florida Senator Mr Rubio has 8% support and the rest closely follow behind him.
Mr Trump has turned his fire to Mr Carson in recent days, attacking him as "low energy" and questioning his Seventh Day Adventist faith.
The two will meet on the stage at Boulder, Colorado, at the Republican debate on Wednesday.
Primary voting begins in February in Iowa, where Mr Carson also leads the polls.
The 64-year-old was criticised this week for comparing abortion to slavery.
The retired doctor has said President Barack Obama's healthcare reform was "the worst thing" since slavery and that the US government is acting like Nazi Germany.
He asserted that being gay is a choice, Muslims aren't qualified to seek the US presidency, the Holocaust could have been prevented if persecuted Europeans owned more guns and - just a few days ago - that the US government should cut off funding to universities that are found to exhibit "extreme political bias".
Mr Carson's comments on abortion have set off yet another cycle of outrage from the left - a reaction that he will likely wear as a badge of honour.
Read Anthony Zurcher's feature in full

The service for the 74-year-old, who died earlier this month a few days after having a stroke, was held at St Peter's Church in Bournemouth.
Stewart's broadcasting career spanned more than five decades.
Fellow DJ Mike Read, actress and singer Anita Harris, and comedian Bobby Davro were among the congregation.
Former snooker player John Virgo also attended.
The service, which was led by the Reverend Dr Ian Terry, included the hymns Amazing Grace and Lord Of The Dance, as well as the song You'll Never Walk Alone.
There was also a farewell address by the Grand Order of the Water Rats - an entertainment industry fraternity - of which Devon-born Stewart was a member.
A private cremation followed the church service.
Dr Terry said the service would "celebrate the life of somebody who just loved life".
"It's a service of thanksgiving for Ed who, through the radio and through the television, gave enjoyment and fun," he added.
Stewart started out as an announcer, film critic and rugby reporter with Radio Hong Kong.
He was one of the first presenters on Radio 1 when it launched in 1967, and a Radio 2 presenter for 15 years - during which he broadcast from the summits of Ben Nevis and Snowdon, Mount Vesuvius volcano in Italy, and also from the Falkland Islands.
On television, Stewart went on to become a regular Top of the Pops presenter in the 1970s and was best known for children's favourite Crackerjack, which he hosted from 1973 to 1979.
He was also a longstanding presenter of children's show Junior Choice, which was last broadcast on BBC Radio 2 over Christmas.
On it he coined his trademark catchphrase: '''Ello darling."
Stewart is survived by his two children, Francesca and Marco, and four grandchildren.
A retired neurosurgeon has knocked Donald Trump off top spot in the Republican presidential campaign.
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Hundreds of people have attended the funeral of former BBC DJ and Crackerjack presenter Ed "Stewpot" Stewart in his hometown.