Endosomes serve as crucial sorting centres that streamline the distribution of cell surface proteins. The early endosome receives traffic from both the plasma membrane (PM) and the Golgi, and orchestrates the redistribution of cargoes for recycling to the PM or through retrograde movement to the Golgi, and for degradation to late endosomes and lysosomes¹. In animal cells and amoebae, Arp2/3 complex-mediated F-actin assembly plays critical roles in many aspects of endosome function, promoting both recycling and degradative trafficking routes². Yeast models, which allowed dissection of the major membrane trafficking routes³, exhibit highly simplified endosomes, as shown in Saccharomyces cerevisiae, where the trans-Golgi network (TGN) functions as recycling endosome⁴. Furthermore, there is no reported role for Arp2/3 complex or F-actin in endomembrane remodelling in yeast cells, which lack Arp2/3 complex activators that function on animal endosomes^5,6. Here, we examine the role of the Arp2/3 complex in the shape and function of fission yeast Schizosaccharomyces pombe endosomes. Through live-imaging and correlative light-electron tomography, we describe endosomes as dynamic tubulo-cisternal compartments, whose morphology requires branched actin, as inhibition of the Arp2/3 complex leads to endosome rounding. Though branched actin primarily localizes to endocytic patches, we show localization of the Arp2/3 complex and F-actin at endosomes for short bursts of time. Remarkably, Arp2/3-dependent actin assembly is critical to allow retrograde trafficking from the endosome to the degradative vacuole. Thus, Arp2/3 complex-dependent actin assembly has a deeply conserved role in shaping and promoting the function of the endomembrane trafficking system.