The district court in the southern city of Gwangju ruled that Mitsubishi Heavy Industries should pay 120 million won ($106,700) in compensation to Kim Young-Ok, 85, and 3.25 million won to a relative of late victim Choe Jeong-Rye, activists and reports said.
Mitsubishi said it would appeal.
It is the second such ruling in four years.
The Korean peninsula was under Japanese colonial rule from 1910-45, when Koreans were banned from using their own language in schools and forced to adopt Japanese names.
Hundreds of thousands of Koreans were forcibly recruited as frontline troops, slave workers and wartime sex slaves known as comfort women.
Issues of history still divide the neighbours, which are both US allies, and complicate their relationship even as they both face threats from nuclear-armed North Korea.
“We welcome this ruling. This is another court victory for the victims and their relatives,” Lee Kuk-Un, who leads a group of activists working for former slave workers, told AFP.
The victims, both in their teens at the time, worked without pay at a Mitsubishi aircraft manufacturing plant in Nagoya in 1944 after being falsely promised they would be able to make money and study in Japan.
Mitsubishi said in a statement it had not yet seen the ruling but would appeal.
“As soon as we obtain the verdict and confirm the details, we want to swiftly go ahead with procedures to appeal the court decision,” the company said.
The case is one of 14 involving more than 1,000 South Korean victims that have been filed against Mitsubishi and several other Japanese firms seeking compensation for wartime slave labour.
The first such ruling came in 2013, when the Gwangju district court ruled in favour of five victims, ordering Mitsubishi to pay each of them sums ranging up to 150 million won.
An appeals court upheld the ruling but Mitsubishi has since taken the case to South Korea’s Supreme Court, which has yet to rule on it.
Japan says colonial-era disputes were settled in 1965, when diplomatic ties were normalised and Tokyo gave Seoul some $800 million in loans and economic aid.
The money helped fund South Korea’s dramatic industrialisation drive.