Billy Sharp also netted in the first half as Rovers extracted revenge for the Tractor Boys' 6-0 thrashing at the Keepmoat Stadium last season.
Diouf broke the deadlock in the 18th minute by heading in James Coppinger's cross, and the left-winger was the architect again moments later, feeding Sharp for his second goal of the week.
Diouf extended the lead, running unchallenged before scoring from distance to put Dean Saunders' side three up going into the interval.
Ipswich boss Paul Jewell introduced Josh Carson at half-time and he responded by reducing the arrears before Michael Chopra ensured a nervy last minute for the visitors by scoring in injury time.
The victory was not enough to move Doncaster off the npower Championship's basement while Ipswich remain marooned in midtable after conceding seven goals in their last two outings.
The visitors started strongly and should have gone ahead after nine minutes.
Brian Stock dinked in a clever ball and Ipswich's defence pushed out leaving George Friend in space. But he failed to control the ball and it rolled out for a goal kick.
Ipswich did not heed the warning and their defence again afforded a Doncaster player plenty of space, and this time Diouf made them pay.
Rovers worked the ball to the left and Coppinger turned before whipping in a cross which was nodded in by the Senegalese striker.
It was deja vu for the hosts as Coppinger sent in another left-wing cross for the second goal six minutes later.
This time it was Diouf's strike partner Sharp who headed home to double the visitors' advantage.
Sharp, who scored against Middlesbrough on Tuesday just three days after the death of his two-day-old son, was given a resounding round of applause from the Portman Road crowd.
Jewell responded by hauling off Colin Healy, replacing him with Lee Martin as he tried to stop the dangerous duo of Coppinger and Friend.
The hosts did have the ball in the net when Chopra converted Reece Wabara's cross, but the former Cardiff striker was adjudged to have been offside.
And the visitors made it three goals from three shots when Diouf struck again six minutes before the break.
He linked up with Sharp before carrying the ball for nearly 30 yards and then planting it into the bottom corner of the net from 25 yards out.
Carson came on for Jimmy Bullard at half-time and the substitution paid dividends when he brought Ipswich back into the game eight minutes into the second half.
The 18-year-old reacted quickest to poke home after Doncaster's goalkeeper Neil Sullivan spilled Tamas Priskin's shot.
Friend then went close twice while, at the other end, Carson was inches away from adding his and Ipswich's second.
Chopra had the ball in the net again when he converted a cross from another substitute Jay Emmanuel-Thomas and although this one counted, it was too little too late for Jewell's side.