Abstract Activated carbon (AC) is a key material in numerous industrial applications, including wastewater treatment, catalysis, personal care products, and pharmaceutical industry. Producing AC from waste biomass is of much interest since the environmental footprint is low and the economics are favorable for most applications. Cashew nut husk (CNH) and cashew nut shell (CNS) are typically considered wastes in the cashew nut processing industry. Synthesis of AC from CNH and CNS allows value addition of these materials and improves the economic prospects of the cashew nut processing industry. Herein, we report the production of AC by carbonization of CNH and CNS using orthophosphoric acid as the activating agent. The results showed that 700 °C is a suitable activation temperature for CNH, whereas 500 °C was enough for activating CNS. The AC samples were extensively characterized by FTIR, PXRD, BET, and FESEM‐EDX analysis. The AC produced from CNH at 700 °C (H‐700) exhibited a very high specific surface area and iodine number of 1511 m 2 /g and 961 mg/g, respectively. Therefore, H‐700 was used for adsorbing methylene blue from water as a model for wastewater treatment. The H‐700 sample showed an adsorption capacity of 520 mg/g with good recyclability up to five cycles.