Mitoxantrone (MIT) is a chemotherapeutic drug widely used for its DNA intercalation and inhibition of topoisomerase. In this work, we show that MIT also affects cytoplasmic RNA–ribosome organization. In human cancer cells, MIT induced stress granules (SGs) that contained large ribosomal subunit proteins, including eL8, together with polyadenylated mRNA. These MIT-induced SGs were different from arsenite-induced SGs: they formed without eIF2α phosphorylation, mTOR inhibition, or 4E-BP1 activity, and they remained stable in the presence of cycloheximide and after drug withdrawal. In vitro assays further demonstrated that MIT promotes ribosome aggregation in a concentration- and salt-dependent manner. Taken together, our results identify a distinct type of ribosome-enriched SGs that form through RNA–ribosome condensation rather than classical translational stress pathways. This mechanism provides a direct example of how a clinically used drug can reorganize cytoplasmic RNA–protein complexes, with possible consequences for mRNA regulation, cancer therapy, and neurodegenerative disease.