Overview
No garment survives from Gojoseon, and no contemporary picture of one. What is known is built from three indirect sources: Chinese records of the peninsula and the peoples around it, the dress of the northern steppe cultures Gojoseon bordered, and the Goguryeo tomb murals painted several centuries later. Those sources agree with one another, which is why the reconstruction below is worth stating — but the structure is far better founded than any detail of it, and that distinction is the honest one to keep.
Key garments and style
The structure was the northern two-piece that underlies all later Korean dress: a close, front-fastening upper garment with narrow sleeves, the ancestor of the jeogori, worn over loose trousers and closed with a sash at the waist. It belongs to the riding cultures of northern Eurasia rather than to the robe traditions of China, and it is why Korean dress kept a separate top and bottom where Chinese dress moved to a one-piece robe.
The physical evidence is indirect but consistent: bronze belt fittings, buttons and dress ornaments from Gojoseon-period sites are objects that only make sense on a belted, fastened garment. Hemp was the ordinary fibre. Furs and hides were worked well enough to be worth exporting — the Guanzi records the state of Qi trading for patterned pelts from Joseon, which is among the earliest notices of the place in any source.
Transition
The Han commanderies, Lelang above all, were planted on the peninsula in 108 BC, and Chinese silk, lacquer and bronze entered the region in quantity; Lelang tombs are full of them. The two-piece structure was not displaced. It passed intact to Buyeo, Goguryeo and the Three Kingdoms, where the Goguryeo murals finally show it plainly — jeogori and trousers, a skirt over them for some women, and a fondness for dotted and banded patterning.