The Quinnipiac poll has Trump at 31 percent leading Texas Senator Ted Cruz at 24 percent.
Florida Senator Marco Rubio is in third place with 17 percent, while no other GOP candidate has double-digit support. Dr. Ben Carson was the closest with 8 percent.
Trump has led almost every poll for months, including both nationwide and primary polls.
His relatively strong numbers in Iowa are boosted by first-time voters, with 40 percent saying they will support him compared to only 22 percent for Cruz.
First-time voters on the Democratic side also pushed Vermont Senator Bernie Sanders to a 3-point lead over Hillary Clinton, 49 to 46 percent.
Sanders held a major lead in the first-time voters, 62 to 35 percent.
“The size of the turnout tonight will likely be the key factor, especially on the Democratic side,” said Peter Brown, assistant director of the Quinnipiac University Poll, in a statement. “High turnouts with lots of new caucus participants likely would mean a good night for Sen. Bernie Sanders, and for Donald Trump.”
Martin O’Malley, the former Maryland governor, is also running but isn’t considered a serious threat.
The survey of 890 likely GOP and 919 likely Democratic caucusgoers was conducted Jan. 25 through 31 via landlines and cellphones. The margin of error was 3.3 points (Republican) and 3.2 points (Democrat).