40 others were injured, a Goof-Gaduud village official, Hassan Bayow, told local radio.
All the victims were civilians, who were there to sell or buy livestock at the time, he told radio Kulmiye.
Bayow said Somali militants al-Shabaab were likely behind the attack because those at the market had refused to pay the group.
Al-Shabaab regularly asks villagers for money handouts in the areas where it is present.
There was no immediate claim of responsibility.