An acid-active exo/endo-chitinase; comprising a GH18 catalytic domain and substrate insertion domain; originating from the thermophilic flamentous fungus Rasamsonia emersonii, was expressed in Pichia pastoris. In silico analysis including phylogenetic analysis, and recombinant production, purifcation, biochemical characterisation, and industrial application testing, was carried out. The expressed protein was identifed by SDS-PAGE as a smear from 56.3 to 125.1 kDa, which sharpens into bands at 46.0 kDa, 48.4 kDa and a smear above 60 kDa when treated with PNGase F. The acid-active chitinase was primarily a chitobiosidase but displayed some endo-chitinase and acetyl-glucosamidase activity. The enzyme was optimally active at 50 °C, and markedly low pH of 2.8. As far as the authors are aware, this is the lowest pH optima reported for any fungal chitinase. The acid-active chitinase likely plays a role in chitin degradation for cell uptake in its native environment, perhaps in conjunction with a chitin deacetylase. Comparative studies with other R. emersonii chitinases indicate that they may play a synergistic role in this. The acid-active chitinase displayed some efcacy against non-treated substrates; fungal chitin and chitin from shrimp. Thus, it may be suited to industrial chitin hydrolysis reactions for extraction of glucosamine and chitobiose at low pH.