The security official in Zinjibar, capital of the southern province of Abyan, said civilians were among the 28 injured.
"The booby-trapped motorcycle exploded as a truck carrying combatants passed by to enter the market," he told AFP.
The blast came hours after an official from the separatist Southern Transitional Council (STC) was killed in Aden by unidentified suspects.
There were no immediate claims of responsibility for either attack.
The STC last year joined a power-sharing unity government, which is fighting Iran-backed Huthi rebels in the country's north.
Its fighters have also played a decisive role in the fight against Al-Qaeda and the Islamic State group, forcing the militants to retreat into rural areas.
Al-Qaeda in the Arabian Peninsula (AQAP) took advantage of the war since 2014 between the government and Huthi rebels, bolstering its presence in southern Yemen.
The United States, which considers AQAP the terror group's most dangerous branch, has targeted it with drone strikes over the past two decades.
