Protesters outside the US embassy in Seoul waved signs saying, “Do not join a war of aggression.”
US President Donald President Trump has pressured key ally South Korea to help in the conflict, which has seen Iran blockage of the strait – a key route for about 20 percent of global energy supplies.
“We cannot send our young people into a sea of death,” chanted demonstrators.
The protest came a day after South Korean authorities said they sent a fact-finding mission to assess the situation in Hormuz, while stressing no decision had been made on any deployment.
MNA