The attack during the morning rush hour in the southern city came a day after a similar bombing killed at least 17 people in its main rail station.
They have raised fears of Islamist attacks on the Winter Olympics, which are being held in the resort city of Sochi next February.
Investigators said they believed a male suicide bomber set off yesterday's trolleybus blast. The blue and white vehicle - powered by overhead electric cables - was reduced to a twisted, gutted carcass and bodies were strewn across the street.
Windows in nearby apartments were blown out by the blast.
One woman, named Olga, who lives nearby, said: "There was smoke and people were lying in the street. The driver was thrown a long way. She was alive and moaning ... her hands and clothes were bloody."
A woman near the scene said: "For the second day, we are dying. It's a nightmare. What are we supposed to do, just walk now?"
Investigators said "identical" shrapnel to that in the rail station indicated the two bombs were linked. There was no immediate claim of responsibility.
On Sunday, investigators initially described the station bomber as a woman from Dagestan, a hub of Islamist militancy on the Caspian, but they later said the attacker may have been a man.
In October, a woman from the North Caucasus blew up and killed seven people on a bus in Volgograd.
It was unclear why the city, which will host soccer matches during the 2018 World Cup, has been hit. However, geography - being close to the restive regions - and its historical significance, may have contributed to it being targeted.
Volgograd has held a place in Russians' sense of national identity since, when known as Stalingrad, its Soviet defenders held off Nazi invaders to turn the course of the Second World War.
City authorities have revived the former name for special occasions because Stalin's image has been somewhat rehabilitated under Mr Putin. However, Stalin remains a hate figure to Chechens, whose nation was deported en masse on the dictator's orders.
In a statement, the Foreign Ministry called on world powers to stand together against "terrorists" and named Chechen Islamist leader Doku Umarov as among those fomenting violence.
The two bombings raise fears of a concerted campaign before the Winter Olympics, which start on February 7 around Sochi, a resort on the Black Sea at the western end of the Caucasus range, 450 miles southwest of Volgograd.
Police said additional officers were being deployed to railway stations and airports nationwide after Sunday's bombing, but the attacks raised questions about the effectiveness of security measures.
The police force in Volgograd has been depleted because some 600 officers were redeployed to Sochi to tighten security around Olympic sites.
Alexei Filatov, a prominent former member of Russia's elite anti-terrorism force, Alfa, said more attacks could be expected before the Olympics. He said cities in southern Russia where the Games were not being held were easier targets than Sochi.
"The threat is greatest now because it is when terrorists can make the biggest impression," he said. "The security measures were beefed up long ago around Sochi, so terrorists will strike instead in these nearby cities like Volgograd."
Mr Putin secured the Games for Russia and has staked his personal reputation on a safe and successful Olympics. This month, he freed jailed opponents, including oil tycoon Mikhail Khodorkovsky and the Pussy Riot punk band, to remove causes for international criticism at the event.