Hypsizygus tessulatus is an edible mushroom species in the family Lyophyllaceae, widely cultivated and consumed in East Asia. It is commonly known as the beech mushroom, bunashimeji, or brown shimeji mushroom. The genus name Hypsizygus derives from Greek roots meaning “high” (hypsi) and “yoke” or “pairing” (zygos), referring to morphological features of the gills and spore-bearing structures, while the species epithet tessulatus comes from Latin meaning “tessellated” or “checkered,” describing the subtle patterned appearance sometimes visible on the cap surface. In Japan, this mushroom is widely marketed as buna-shimeji (ブナシメジ), referring to its association with beech trees (buna).

In the wild, Hypsizygus tessulatus grows on decaying hardwoods, especially beech trees, forming clustered fruiting bodies that emerge from logs or stumps. The mushrooms have small convex caps that range from light to dark brown, slender white stems, and tightly spaced gills underneath the cap. Modern cultivation techniques have made it a common commercial mushroom grown in controlled environments using sawdust-based substrates. Cultivated clusters are harvested as dense bundles of slender stems with small caps, making them convenient for culinary use. There is a UV-induced mutated strain of this species which causes it to be completely white and are sold under the market name "bunapi".

As a food source, Hypsizygus tessulatus is valued for its firm texture and mild, slightly nutty flavor that develops more fully when cooked. The mushrooms are typically sautéed, stir-fried, grilled, or added to soups, stews, and hot pot dishes. In Japanese cuisine they are frequently included in miso soup, noodle dishes, and mixed mushroom preparations. Raw mushrooms may have a faint bitterness, which disappears during cooking as heat alters certain flavor compounds.

Nutritionally, Hypsizygus tessulatus provides dietary fiber, protein, and several vitamins and minerals including B vitamins, potassium, and phosphorus. Like many edible fungi, it contains bioactive compounds such as polysaccharides (notably β-glucans), ergothioneine, and phenolic antioxidants. These compounds have been investigated for potential health-related effects including antioxidant activity, immune modulation, and metabolic regulation in experimental studies. The mushroom is also low in fat and calories, making it a popular ingredient in balanced diets.