His absence was mocked by his rivals, who tried to fill the space vacated by Mr Trump’s boycott by attacking each other on immigration and other issues.
Mr Trump decided to withdraw after Fox News refused to drop debate host Megyn Kelly, whom he accused of bias.
The billionaire held a rally nearby, in honour of war veterans, that threatened to overshadow the debate itself.
On Monday, voters in Iowa are due to pick their presidential nominee for each party.
Days ahead of that critical test, Mr Trump’s absence on the stage in Des Moines was keenly felt by his seven rivals in the race to be Republican presidential nominee.
Texas Senator Ted Cruz addressed it with humour in the opening minutes by throwing mock insults at his rivals.
“I’m a maniac and everyone on this stage is stupid, fat and ugly, and Ben [Carson], you’re a terrible surgeon,” he said.
Former Florida Governor Jeb Bush also poked fun at the hotel tycoon, his chief tormenter in previous debates, by saying how much he missed him.
Elsewhere in Des Moines at the same time, Mr Trump led a raucous rally in honour of the country’s war veterans.
“When you’re treated badly, you have to stick up for your rights,” he said, referring to his row with Fox.
The broadcaster released a statement that said Mr Trump offered to appear at the debate if Fox contributed $5m (£3.4m) to his charities, but they refused.
Data released by Google after the debate suggested that search interest in Mr Trump still far surpassed the other candidates.
Many observers on social media thought the event was duller without the brash New Yorker.
But others remarked how the absence of his dominating personality helped other candidates to blossom.
Some of the night’s most heated moments came during exchanges about immigration.
Mr Cruz and Mr Rubio were both forced to explain video clips of previous statements that appeared to be at odds with their hardline campaign pledges.
They then turned on each other, with Mr Rubio saying immigration was “the lie that Ted’s campaign is built upon”.
An Iraq war veteran who came to the US from Mexico as a child appeared via YouTube to tell the candidates that “some of the comments in this campaign make us question our place in this country”.
Mr Bush applauded Dulce Candy and said “we should be a welcoming nation”.