Police say the explosion Saturday morning in Yala province also set five buildings on fire in the city center.
Police Col. Narin Busaman says the suspected militants set off the remote-controlled bomb, which was hidden in a motorcycle, when a group of patrolling soldiers passed by the city's commercial area.
He says five soldiers and 15 civilians were injured. Police have not identified the dead victim.
More than 5,000 people have been killed in Thailand's Muslim-dominated southernmost provinces since an Islamist insurgency erupted in 2004.
Obama arrives in Thailand on Sunday. He'll also visit Myanmar and Cambodia on his trip.