Authigenic pyrite is prevalent in the sedimentary deposits of Brunei Darussalam, occurring in well-defined morphologies such as cubes, pyritohedra, octahedra, and framboidal textures. These sulphide minerals are commonly associated with clay-rich matrices and lignitic horizons, indicative of anoxic depositional environments enriched in organic matter that facilitate pyrite formation. Upon exposure, pyrite undergoes oxidation, often accelerated by microbial activity, producing native sulphur and gypsum. Pyrite oxidation releases Fe²⁺ which is oxidised to Fe³⁺ subsequently forming Fe-oxides such as goethite, hematite and limonite. These phases contribute to the reddish pigmentation of soils and act as cementing agents in arenitic sands, forming ferricrusts. The geochemical alteration of pyrite significantly lowers the pH of surrounding soils, resulting in acidic conditions that pose serious challenges to agriculture and aquatic ecosystems due to the discharge of acidified waters. This study elucidates the mineralogical evolution and environmental consequences of pyrite-bearing sediments in Brunei, underscoring the need for targeted mitigation strategies in acid sulphate soil landscapes.