Bolshoy Lyakhovsky Island (Russian: Большой Ляховский остров), or Great Lyakhovsky, is the largest of the Lyakhovsky Islands belonging to the New Siberian Islands archipelago between the Laptev Sea and the East Siberian Sea in northern Russia. It has an area of 5,156.6 km² (1,991.0 sq mi), and a maximum altitude of 311 m (1,020 ft) (Emy Tas).

The peninsula projecting towards the west of the island is the Kigilyakh Peninsula (Poluostrov Kigilyakh).

Off Bolshoy Lyakhovsky Island's southwestern cape lies a small islet called Ostrov Khopto-Terer.

The Lyakhovsky Islands are named in honour of Ivan Lyakhov, who explored them in 1773.

Bolshoy Lyakhovsky Island consists of highly folded and faulted Precambrian metamorphic rocks and turbidites; Mesozoic turbidites and igneous rocks; and Cenozoic sediments. Exposed in southeastern part of this island, the older Precambrian. Early Proterozoic, metamorphic rocks consist of schists and amphibolites. Small exposures of Late Proterozoic schistose, quartzose sandstones and phyllitic, sericite-quartz schist (turbidites) also occur in the southeastern part of Bolshoy Lyakhovsky Island. The bulk of this island consists of Late Jurassic to Early Cretaceous turbidites composed of interbedded fine-grained sandstones, thinly bedded siltstones, and argillites. The Precambrian and Late Jurassic to Early Cretaceous rocks are intruded by Late Cretaceous granites and granodiorites.

A blanket of unconsolidated Cenozoic sediments blankets most of Bolshoy Lyakhovsky Island. These sediments include Paleocene to Eocene colluvial, alluvial, and deltaic gravels, sands, clays, and coals and Oligocene to Miocene alluvial, lacustrine, deltaic, and nearshore marine sands and clays that contain beds and lenses of gravel. Overlying these sediments are Pliocene to Pleistocene colluvial, alluvial, and nearshore marine sands, silts, and clays that contain occasional gravel layers. The nearshore marine sediments contain the shells of marine mollusks and pieces of lignitized wood. Thick permafrost characterized by massive ice wedges has developed in these sediments. Contrary to the interpretations of Baron von Toll and earlier geologists, glacial tills and related sediments are completely absent within Bolshoy Lyakhovsky Island.

In Soviet times on the Kigilyakh Peninsula Vladimir Voronin, then in charge of the Bolshoy Lyakhovsky Polar Station, was shown a large standing rock which had been heavily eroded and which gave name to the peninsula. The word Kigilyakh means "stone man" in the Yakut language.

The southern sea cliffs of Bolshoy Lyakhovsky Island, which were first studied by Baron von Toll, expose a complex sequence of fossiliferous Late and Middle Pleistocene floodplain, eolian, lake, and solifluction sediments cemented by permafrost. In the 1990s and 2000s, a Russian-German multidisciplinary team, which included the Alfred Wegener Institute for Polar and Marine Research and Permafrost Institute, Russian Academy of Sciences, studied the cryolithology, geochronology, ground ice geochemistry, paleobotany, paleontology, sedimentology, and stratigraphy of these sediments. The age of these sediments were studied in great detail using radiocarbon dating of bones, ivory, and plant remains; optically stimulated luminescence dating of bone-bearing sediments; and uranium-thorium dating of associated peat.