The forward made a return to the side after serving out a six-match ban due to an alleged spitting incident in their tie against Birmingham City in April - an action that was punished in May after an appeal from the club was rebuffed.
In his first game of the season, the 25-year-old found the net shortly after replacing Matt Taylor in the 73rd minute.
Prior to the effort from the former Angers striker, Charles Mulgrew had earlier opened the scoring for the visitors before Josh Brownhill levelled for the Robins.
Marley Watkins then gave Lee Johnson’s men the lead for the first time in the encounter before Diedhiou increased the lead.
Diedhiou will be looking to continue the impressive display in their next league game against Sheffield United on September 15.