Goals from Conor McLaughlin and Devante Cole stunned the Blades and extended the Cod Army's unbeaten run in the league to an impressive 11 games, moving them to within six points of second-placed Scunthorpe.
It was a thoroughly deserved three points as Uwe Rosler's men impressed from the off, taking the lead in the 20th minute when McLaughlin drilled in Bobby Grant's cross that was meant as a shot at the far post.
They should have been 2-0 up at the break, but Simon Moore denied Wes Burns with an instinctive save when the striker went through on goal.
Billy Sharp was United's main threat as they chased an equaliser early in the second half, but Fleetwood killed the game when Cole latched on to Ethan Ebanks-Landell's weak header to lob Moore.
The Blades, who paraded new signings James Hanson and Jay O'Shea before kick-off, pressed for a way back into the game but could not find it and have now taken just one point from their last three games.
Report supplied by the Press Association.
Match ends, Sheffield United 0, Fleetwood Town 2.
Second Half ends, Sheffield United 0, Fleetwood Town 2.
Foul by John Fleck (Sheffield United).
Conor McLaughlin (Fleetwood Town) wins a free kick in the attacking half.
Substitution, Sheffield United. Matt Done replaces Daniel Lafferty.
Attempt blocked. Caolan Lavery (Sheffield United) right footed shot from the centre of the box is blocked.
Attempt missed. Bobby Grant (Fleetwood Town) left footed shot from outside the box is just a bit too high.
Attempt blocked. Ashley Hunter (Fleetwood Town) right footed shot from the right side of the box is blocked.
Ethan Ebanks-Landell (Sheffield United) wins a free kick in the attacking half.
Foul by Cian Bolger (Fleetwood Town).
Billy Sharp (Sheffield United) wins a free kick in the defensive half.
Foul by Cian Bolger (Fleetwood Town).
Foul by Caolan Lavery (Sheffield United).
Cian Bolger (Fleetwood Town) wins a free kick in the attacking half.
Attempt saved. Ethan Ebanks-Landell (Sheffield United) header from the centre of the box is saved in the centre of the goal.
Corner,  Sheffield United. Conceded by Cian Bolger.
Attempt blocked. Caolan Lavery (Sheffield United) right footed shot from the centre of the box is blocked.
Corner,  Sheffield United. Conceded by Cian Bolger.
Caolan Lavery (Sheffield United) wins a free kick in the defensive half.
Foul by George Glendon (Fleetwood Town).
Jack O'Connell (Sheffield United) wins a free kick in the attacking half.
Foul by David Ball (Fleetwood Town).
Foul by Billy Sharp (Sheffield United).
Ashley Eastham (Fleetwood Town) wins a free kick on the left wing.
Substitution, Fleetwood Town. George Glendon replaces Markus Schwabl.
Attempt missed. Cian Bolger (Fleetwood Town) header from the centre of the box misses to the left following a corner.
Corner,  Fleetwood Town. Conceded by Stefan Scougall.
Substitution, Fleetwood Town. David Ball replaces Devante Cole.
Attempt missed. Billy Sharp (Sheffield United) header from the centre of the box is just a bit too high.
Ethan Ebanks-Landell (Sheffield United) wins a free kick in the attacking half.
Foul by Ashley Hunter (Fleetwood Town).
Goal!  Sheffield United 0, Fleetwood Town 2. Devante Cole (Fleetwood Town) right footed shot from the centre of the box to the centre of the goal.
Devante Cole (Fleetwood Town) is shown the yellow card.
Kieron Freeman (Sheffield United) wins a free kick on the left wing.
Foul by Amari'i Bell (Fleetwood Town).
Substitution, Fleetwood Town. Ashley Hunter replaces Wes Burns.
Attempt saved. Billy Sharp (Sheffield United) right footed shot from outside the box is saved in the bottom left corner.
Attempt missed. Billy Sharp (Sheffield United) right footed shot from the left side of the six yard box misses to the right.
Foul by John Fleck (Sheffield United).
Markus Schwabl (Fleetwood Town) wins a free kick in the attacking half.
Fleetwood highlighted their credentials as automatic-promotion candidates in League One by beating table-toppers Sheffield United.