Pol Col Suwat Wongpaibul, chief of Bannang Sata police station, said three of them were in a critical condition.
The explosion occurred at about 10.50am on a section of the Bannang Sata-Than To road near Ban Tao Poon school. All the victims were rushed to the district hospital.
Police investigators said the 10kg homemade bomb was hidden inside a parked motorcycle.
They believed attackers detonated the explosives as soldiers from Yala's No.15 special forces unit were passing by on motorcycles.
Several cars, including a military GMC truck near the explosion scene, were damaged by the impact.
Shortly after the bombing, police detained a suspect identified as Wae-Useng Waesama, 25, of Ban Buerapeh in Bannang Sata district.
He was taken to Bannang Sata police station for questioning.
In Yaha district, a five-man teacher protection unit escaped unharmed when a bomb exploded.
The soldiers were patrolling when the bomb planted on the Tachi-Yaha road was detonated. No-one was injured.
In neighbouring Pattani province, two teacher protection volunteers were killed and two others wounded in an explosion yesterday afternoon in Yaring district.
The two dead victims were identified as Manit Padsichae, 36, and Prateep Kajjakul, 28. The wounded were identified as Somchok Musin, 43, and Sarot Wongsa, 39.
The explosion took place as the four were taking a break in a hut while working in a coconut plantation in tambon Talorkapo. The bomb was planted near their break area.
The four men were part of a teacher protection unit and they usually stopped at the hut during breaktimes.
In another development, police were investigating the murder of Pol Sen Sgt Maj Charoen Kaewmani, 64, a retired police officer, in Panare district on Monday night.
The police officer was waiting for his wife in front of a house when an unidentified group of attackers emerged and fired a gun at him at close range.
Pol Sen Sgt Maj Charoen was pronounced dead at the scene.
Two more shooting attacks were also reported on Monday in Nong Chik district.
The first incident was reported at about 6pm and the second about an hour later. In the first attack, Nibong Songserm, 34, was shot in the leg by a group using AK and M16 rifles.
The man was riding his motorcycle when he was attacked.
In the second incident, Phornthep Rattana, 34, a village defence volunteer in tambon Pulu Puyo, was seriously injured in a drive-by shooting as she travelled on her motorcycle with another woman riding pillion.
Meanwhile, the Southern Border Provinces Police Operation Centre said it was stepping up security measures in the deep South ahead of the Muslim holy month of Ramadan, which starts on Aug 1.
