A graphene template provides a unique opportunity to combine nanomaterial building blocks on its surface for versatile optoelectronic applications. Herein, a low-cost, facile, and non-toxic nanocomposite of 2D metallic MXene (MX) and Prussian blue (PB) on a graphene template is reported. The MX nanosheets are exfoliated from the MAX phase via HF etching and are decorated with semiconducting PB by using a novel one-step in situ method. This conjugation effectively anchors PB to MX, thereby enhancing the light absorption and providing a significant 19-fold increase in fluorescence quenching due to direct charge transfer. The PB@MX nanocomposite is then coated onto the graphene template to fabricate a heterostructured photodetector (PD) that exhibits enhanced photocurrent and highly sensitive broadband photodetection across the 325–808 nm range. The nanocomposite-based PD exhibits a remarkable photocurrent of 47.97 μA, a high photoresponsivity of 14 880 A W−1, a detectivity of 3.72 × 1012 cm Hz1/2 W−1, and a normalized gain of 4.15 × 10−4 m2 V−1 under 450 nm illumination at 31.84 μW cm−2 and 1 V. These values represent the best performance reported to date for this class of nanohybrids, thus highlighting the potential of this PD for real-world electronic applications.