In this study, the zinc oxide-decorated graphitic carbon nitride composite (ZnO/gC3N4 – ZCN) was synthesized for photocatalytic degradation of organic dyes in aqueous media. Particularly, ZnO was fabricated by the calcination method with the assistance of Garcinia mangostana pericarp extract as a stabilizer and reducing agent, while graphitic carbon nitride (gC3N4 – CN) was prepared by the calcination method and exfoliated using a strong oxidizer mixture. Characterization results showed ZnO possesses a spherical shape with an average diameter of 30–80 nm and is distributed evenly on the CN sheets. Subsequently, ZCN was subjected to photodegradation towards methylene blue (MB), which indicated that under optimal conditions of 50 mg of catalyst, pH 9, and 10 mg/L of MB concentration, the overall efficiency reached up to 99.16 % after 180 min of visible irradiation. Additionally, ZCN also possesses the degradation ability for other organic dyes, for instance, malachite green (MG) and methyl orange (MO) with an overall efficiency of 96.42 and 57.57 %, respectively. Moreover, the radical scavenging experiments elucidated that the major responsible component for the photodegradation was the photogenerated holes, while other radicals also facilitated the process. Besides, the reusability of the material was excellent as only 10 % of the yield decreased after five cycles. The aforementioned findings reveal the notable potential applications of the ZCN material in the environmental remedial field. Display omitted •The ZnO/gC3N4 composite was biosynthesized with the aid of sonication.•The spherical ZnO distributed evenly on the gC3N4 exfoliation sheet.•The composite exhibited great methylene blue photodegradation under visible light.