Crowds sang the Christian hymn “Amazing Grace” as medics treated the woman at the event in Missouri on the eve of the midterm elections.
Mr Trump stood in silence on the stage as the rally was halted for about 10 minutes.
He had been attacking Hillary Clinton and recalling his victory in the 2016 presidential election when the woman collapsed.
After noticing a commotion in the crowd at Cape Girardeau’s Show Me Centre, he said: "Is there a doctor in the house, please? Doctor? Please. Thank you.”
As medics converged on the woman, he asked the audience to “say a little prayer”.
The crowd began singing “Amazing Grace” a short time after.
The woman was taken out of the audience but was reported to be unharmed.
"That was really something," Mr Trump said as he returned to the podium. "I want to just thank everybody for the way you behaved. That was beautiful."
Medical incidents are fairly common at political rallies in the US, where security often require audience members to arrive hours before they begin.
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