Pack breathable layers and good walking shoes year-round; add rain gear for June-September, and bring a warm coat, gloves, and thermal layers for winter travel.
Clothing
Lightweight base layers (breathable for humid summers)
Long-sleeve shirt (sun, subway AC, and layered wear)
T-shirts (quick-dry preferred)
Sweater or fleece (spring/fall evenings)
Packable insulated jacket (winter or late autumn)
Rain jacket or compact umbrella (summer monsoon season)
Comfortable pants/jeans (for city walking and cooler months)
One smarter outfit (nice restaurants, cafes, nightlife)
Socks and underwear (more pairs than days for humid weather)
Sleepwear (indoor heating can make rooms feel dry in winter)
Footwear
Comfortable walking shoes (lots of stairs and transit walking)
Waterproof sneakers or shoes (rainy season)
Sandals or slides (summer and hostel/guesthouse use)
Warm boots (winter travel, especially Seoul and northern regions)
Extra insoles or blister pads (long walking days)
Easy slip-on shoes (for traditional floor-seating places)
Toiletries & Health
Sunscreen SPF 50+ (strong UV, even in cooler months)
Lip balm (dry winter air)
Moisturizer/hand cream (indoor heating and dry seasons)