Dimarco opened the scoring in the 20th minute in front of 50,325 fans in brisk conditions, before 23-year-old Diouf doubled Inter's advantage in the second half.
Francisco Conceicao produced a late highlight for Juventus when he finished superbly in the 85th minute.
The 28-year-old Dimarco is rounding into top gear, having also scored Inter's only goal in their 1-1 draw with rival AC Milan on the same ground on Wednesday.
Having impressed in goal against AC Milan, new signing Ivan Provedel kept a clean sheet before being substituted midway through the second half.
Juventus rued sloppy finishing and their methodical style of play did not trouble Inter, who bagged the Italian domestic double last season.
Juventus, who finished sixth in last season's Serie A, entered unbeaten in the pre-season, which included beating Chelsea 1-0 in Hong Kong on Wednesday.
Rising Turkish winger Kenan Yildiz started, having come off the bench against Chelsea amid reports that he has been battling a knee injury.
In what has become their brand under coach Luciano Spalletti, Juventus were patient in the early exchanges and played along the flanks.
But Inter were not flustered and made their move when Dimarco tapped in a sharp cross from Ange-Yoan Bonny.
Inter took control only to be denied by goalkeeper Michele Di Gregorio, who completed several outstanding saves.
After a quiet start, Yildiz attempted to spark his side before the interval, but his bullet from outside the box was off target.
Both coaches rang the changes at the break, including Dimarco after he played the entire game against AC Milan.
Inter continued to press and were rewarded in the 62nd minute when Diouf hit the back of the net after finishing off a crisp pass from Pio Esposito, who is reportedly being courted by Manchester United and Arsenal.
Juventus attempted a late rally when Conceicao scored, but it wasn't enough.
They round out the Perth friendlies against Serie B team Palermo on August 11th, while Inter wrap up their pre-season against Real Betis in Bari on August 15th.
The Serie A season starts on August 22nd.