Boletus semigastroideus is a rare, secotioid (partially gasteroid) bolete, endemic to New Zealand, and phylogenetically part of the porcini (Boletus s.str.) clade. It differs from typical boletes by having a closed, pouch-like fruitbody that does not open to release spores, relying instead on bird and insect dispersal.

Cap: Depressed‑globose to sub‑turbinate, up to 30 mm tall x 40 mm wide. Apex bluntly rounded; base abruptly truncated and slightly excavated. Peridium initially white, becoming ochraceous with age; smooth, slightly viscid, and irregularly areolate as the gelatinised outer layer cracks. Base finely pubescent and papillate.
Stipe: Short, tapering, up to 20 mm long x 10 mm thick. Initially white, becoming greyish or ochraceous. Smooth, dull, hollow, narrowing internally to a white columella 2-3 mm thick.
Gleba: Ferruginous (rust-brown). Composed of lenticular chambers up to 1 mm long.