Lee Johnson’s men were searching for their sixth victory of the season but were met by the resilient defence of the Tigers.
However, the 25-year-old forward broke the deadlock with his 93rd-minute header, similar to last weekend outing against Brentford where Niclas Eliasson netted a late winner.
The result helped the Robins move to the eight spot in the log after garnering 22 points from 14 games.
Relieved to secure the maximum points, the former Angers attacker has praised his teammates on the performance.
“It was very important to win tonight,” Diedhiou told club website.
“Mentally it was important because we won in the last minute, the same as we did against Brentford.”
“I waited to see if the ball was going in and when I could see it was a goal I just ran [to celebrate]
“Yes, it wasn’t easy. We worked very hard today. Congratulations go to the boys and we just tried to keep going. We wanted to win today.
“The confidence comes if we play together. We tried to work hard and communicate and it’s very important to win, it’s good for us.”
Diedhiou who has scored three goals in nine league appearances will hope to continue the impressive form in front of goal when Bristol host Stoke City on Saturday.