Trump has the support of 38% of Florida's likely GOP primary voters, compared to 30% who back Rubio, 17% for Texas Sen. Ted Cruz and 10% for Ohio Gov. John Kasich.
The snapshot of Florida comes before the state's crucial 99-delegate, winner-take-all primary set for March 15.
It's a must-win for Rubio, who has fallen behind Trump and Cruz in the delegate race after taking only two of the first 20 Republican contests.
The poll undercuts the argument that Rubio could win if he faced Trump head-to-head: The real estate mogul leads a one-on-one race, 47% to 45%.
The poll shows Rubio leading among Floridians who have already cast their ballots in early voting, while Trump does better with those who haven't voted yet.
Rubio also leads among those who live in the southern portion of Florida, while Trump does better in central and northern Florida.
Monmouth surveyed 403 Florida voters likely to participate in the GOP primary from March 3-6. The poll's margin of error is plus or minus 4.9 percentage points.
The poll comes as Trump's campaign ramps up attacks on Rubio in paid ads. A television spot released Monday -- "Corrupt Marco"
-- targets Rubio's personal finances and Senate voting record.
In response the Rubio campaign noted Trump's past campaign donations to the man Rubio beat six years ago to win his Senate seat, Charlie Crist, who served as a Republican and later became a Democrat.
"First, Donald Trump gave big bucks to Charlie Crist in 2010 and now he's recycling the same false and negative attacks about Marco," said Rubio spokesman Joe Pounder. "Charlie Crist's and Donald Trump's con didn't fool Floridians in 2010 and it won't fool them now. These attacks didn't even stop Trump himself from saying Mitt Romney should pick Marco as his vice president just two years later."