The bomber rammed his motorcycle into the bus at Chuhr Chowk, a crowded commercial area located near the cantonment in Rawalpindi.
Quoting senior police office Nasir Durrani, Geo News reported that at least six people were killed in the blast.
Twelve others were also injured in the blast, another channel Express 24/7 said and beamed footage of security personnel collecting body parts of the bomber and components of the motorcycle.
Some media reports said the bus was carrying employees of an intelligence agency. Several other vehicles were also damaged in the attack.
Security forces cordoned off the area and forensic experts scoured the site for clues. Security was further tightened in Rawalpindi and Islamabad following the attack.
Both cities were placed under an extensive security blanket some weeks ago in the wake of a wave of bombings and suicide attacks across Pakistan. The attacks have been blamed on the Pakistani Taliban.