The blast comes a day after 17 people were killed in another suicide attack at the central station in the city.
Security has been tightened at railway stations and airports across Russia.
Moscow is concerned militant groups could be ramping up violence ahead of 2014 winter Olympic Games in Sochi.
According to the BBC, the Olympic venue is close to Russia's volatile north Caucasus region.
Volgograd lies about 900km (560 miles) south of Moscow, 650km north of the North Caucasus and 700km north-east of Sochi.
The latest explosion took place near a busy market in the city's Dzerzhinsky district.
Maksim Akhmetov, a Russian TV reporter who was at the scene of the blast, said the trolleybus was packed with people going to work in the morning rush hour, the report said.
He described the scene as 'terrible', adding that the bus was 'ravaged' and that there were 'bodies everywhere, blood on the snow'.
A spokesman for Russia's Investigative Committee said both explosions were now being treated as acts of terrorism, the report added.
--ANI (Posted on 30-12-2013)