That makes for eight of the 13 people who have been trapped under increasingly risky conditions since late last month.
Narongsak Osotthanakorn said the boy's rescue involved numerous same divers who brought the four boys out Sunday.
He added that the rescue operation took nine hours, two hours quicker than the mission on Sunday that saw the first four boys rescued from the cave complex.
But he could not confirm if all five would come out today.
Officials last week said they would bring the fittest people in the group out first, but Narongsak later said that whoever was ready first would be escorted out.
Six hours after operations restarted on Monday morning, rescue workers carried the first stretcher from the mouth of the cave to a waiting ambulance, which then sped away with its lights flashing.
The boys are being guided out of the cave by two divers with oxygen tanks.
In all, 12 boys and their coach had been stuck in the Tham Luang cave network since June 23, when water levels partially flooded the escape routes.
The team became trapped on June 23 when they set out to explore the cave after soccer practice and rains flooded the tunnels, trapping them inside.
Dozens of foreign divers and other experts from around the world were brought in to help the rescue effort, working alongside Thai Navy SEALs.
The death Friday of a former Thai navy SEAL, Saman Gunan, underscored the risks.
The experts say if the boys are not rescued over the next few days, they may have to wait inside the cave for months before the rains ease up and another rescue attempt is made.
Each had to wear an oxygen mask to enable normal breathing, authorities said.
The rescuers are racing against time, as more rain is forecast, and the extra water would significantly complicate matters.
There was no immediate word on their conditions.
"The operation went much better than expected", mission head Narongsak Osottanakorn said on Sunday.
One boy asked for a fried chicken dinner.
Even their mothers and fathers have been unable to visit them.
At the hospital in Chiang Rai, green canvas sheets had been put up to block the entrance from view.
The names of the rescued boys were not released.
Thai public television has aired live video of a medivac helicopter landing close to a hospital in the city of Chiang Rai, near the site of the cave where a youth soccer team has been trapped for more than two weeks. Two of them possibly have a lung infection but all eight are generally "healthy and smiling", he said.
The disease is caused by bat and bird droppings and can be fatal if untreated and allowed to spread to other parts of the body.
He also urged the media to behave respectfully. A relative of one member of the team said that the boys' families had agreed to remain at the cave until all the boys and the coach were brought out, reports CNN.
"We've used all the oxygen", Osatanakorn told reporters.
"I'm hoping for good news", he said. One was that it was unknown how safe and dry the area where they had taken shelter would stay as Thailand's rainy season, which lasts until at least late October, picks up pace.