Core–shell p–n heterojunction nanowires (NWs) hold great promise for optoelectronic applications due to the large effective junction area and minimized carrier recombination. However, the rational design and synthesis of core–shell heterojunction NWs are hindered by the complex fabrication process and the potential damage to core NWs during shell deposition. Herein, we demonstrated the fabrication of Ge–CdS core–shell heterojunction NWs via a facile atomic layer deposition (ALD) technique. The ALD technique allowed the conformal deposition of polycrystalline CdS shells on single-crystalline Ge NWs with high uniformity and controllable thickness. The heterojunction NWs exhibited excellent diode characteristics, with a pronounced photoresponse under light illumination. Significantly, owing to the existence of a built-in electric field, the heterojunctions could serve as self-driven photodetectors, with a high photosensitivity of 18 000%, which is remarkably much better than previous reports on Ge NW photodetectors. The complementary bandgaps of Ge and CdS also ensured that the device had a capability for broadband detection from visible to infrared light. This study represents an important advance in fabricating core–shell heterojunction NWs for high-performance optoelectronic applications.