The Pink Light campaign was tested by 500 pregnant women in the city of Busan over a five-day period.
The women carried sensors that activated pink lights by priority seats on the Busan-Gimhae Light Rail service.
However some might be embarrassed by the extra attention, said podcaster and tech journalist Ellie Gibson.
The sensors have six months of battery life and must be carried outside a bag for maximum signal strength, although they are not waterproof, the Pink Light website (in Korean) says.
The project was a collaboration between the city council and local businesses.
"Consideration for pregnant women should prevail and they should be able to use public transportation more easily and conveniently with this policy," said Suh Byung-soo, Busan's mayor.
"Women should be able to use city facilities easily even when they are expecting."
Many pregnant women report difficulties getting seats on public transport, while passengers say they can't always tell whether a woman is pregnant and don't want to cause offence.
In the UK, Transport for London offers a free pin badge to pregnant travellers, which reads "Baby on board".
The Pink Light campaign "feels a bit like an overly complicated solution to a simple problem," said Ms Gibson, from parenting podcast Scummy Mummies.
"I found the level of attention embarrassing when I wore the 'Baby on board' badge - I'm not sure how comfortable I would feel with pink lights going off when I got on a train."
A bluetooth system that alerts underground train users to give up their seats for pregnant travellers has been trialled in South Korea.