The microtubule motor dynein mediates polarised trafficking of a wide variety of organelles, vesicles and macromolecules. These functions are dependent on the dynactin complex, which helps recruit cargoes to dynein's tail and activates motor movement. How the dynein‐dynactin complex orchestrates trafficking of diverse cargoes is unclear. Here, we identify HEATR5B, an interactor of the adaptor protein‐1 (AP1) clathrin adaptor complex, as a novel player in dynein‐dynactin function. HEATR5B was recovered in a biochemical screen for proteins whose association with the dynein tail is augmented by dynactin. We show that HEATR5B binds directly to the dynein tail and dynactin and stimulates motility of AP1‐associated endosomal membranes in human cells. We also demonstrate that the Drosophila HEATR5B homologue is an essential gene that selectively promotes dynein‐based transport of AP1‐bound membranes to the Golgi apparatus. As HEATR5B lacks the coiled‐coil architecture typical of dynein adaptors, our data point to a non‐canonical process orchestrating motor function on a specific cargo. We additionally show that HEATR5B promotes association of AP1 with endosomal membranes independently of dynein. Thus, HEATR5B co‐ordinates multiple events in AP1‐based trafficking. Synopsis: How the dynein motor and its activator dynactin recognise a wide variety of cellular constituents and translocate them along microtubules is unclear. This study demonstrates that the HEATR5B protein has a selective and evolutionarily conserved role in promoting dynein‐dynactin complex‐dependent trafficking of adaptor protein‐1 (AP1)‐associated endosomal membranes. A novel biochemical screening approach identifies new interactors of the dynein tail complex.HEATR5B associates with dynein and dynactin, thus promoting the trafficking of AP1‐associated endosomal membranes in human cells.The Drosophila HEATR5B orthologue is an essential gene that stimulates dynein‐based delivery of AP1‐bound endosomes to the Golgi apparatus.HEATR5B also promotes association of AP1 with membranes independently of dynein and dynactin. [ABSTRACT FROM AUTHOR]