I am a Nigerian and making a round trip for holiday from Lagos to East Timor, with a transit at Incheon airport, South Korea. Do i need a transit visa?
You don’t need a visa if you transit within 24 hours and you hold valid documents for your next destination.
Per TIMATIC, the database used by airlines:
TWOV (Transit Without Visa): Visa required, except for Passengers transiting through Seoul (ICN) with a confirmed onward ticket for a flight to a third country within 24 hours. They must stay in the international transit area of the airport and have documents required for the next destination.