"Should I store my bag at the airport or in the city?" comes down to one thing: where you're spending the day. If you're out sightseeing, storing in the city near your route almost always beats a round trip to the airport just to drop a bag.
Airport or city — which is smarter?
Both Incheon and Gimpo have coin lockers and staffed storage counters, and they're the obvious pick for a long layover, or a late flight after you've already left your accommodation. But for a normal last day, hauling your suitcase out to Incheon at 9am and coming back defeats the point. Store it in the city, closer to where you'll be, and grab it on the way out.
Lockers and desks at the airport
If the airport genuinely is your base for the day, both terminals have coin lockers and manned storage. Rates run a little higher than the subway lockers, but you skip the transfer — which suits early arrivals waiting on a hotel check-in.
The AREX route trick
The AREX airport railroad runs through Seoul Station and Hongik Univ on the way to Incheon, and also serves Gimpo. Store your bag at whichever you'll pass through, do your day, then ride straight to the airport with it — no doubling back. That one habit saves the most hassle.
Or have your bag delivered
The staffed T-Luggage desks offer same-day delivery to the airport. Hand over the suitcase in the morning and it meets you there — the whole day, and the trip out, completely hands-free. Worth it for a big case or a tight schedule.
Gimpo vs Incheon
Gimpo is much closer to central Seoul and handles domestic plus some short-haul international flights; Incheon is the main international hub. Check which you're flying from before you pick a spot — for Incheon, storing along the AREX line makes the most sense. The full guide ties it together.