Layered IV-VI diluted magnetic semiconductors (DMSs) have exhibited fascinating ferromagnetism down to atomic layers, but their relatively low Curie temperature (TC, ≤200 K) significantly hinders their practical application. In this work, Mn-doped GeSe (GeMnSe) DMSs with high-TC ferromagnetism (FM) are synthesized by chemical vapor deposition. As the Mn concentration varies, the obtained samples exhibit various structures including single-crystalline nanocombs (SC-NCs), polycrystalline nanoparticles (PC-NPs) and amorphous nanoaggregates (a-NAs). All the samples exhibit FM, and their TC and saturation magnetization (MS) are correlated to their structures. Notably, GeMnSe SC-NCs show a record high TC of 309 K and a record strong magnetic moment of 4.37μB/Mn compared to all the previously-reported IV-VI DMSs. Further analysis shows that the FM originates from the synergetic effect of the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction, the crystalline order and the shape anisotropy in the samples. Our results provide a panorama of nanostructure-dependent FM in GeSe-based DMSs and suggest a peculiar playground for investigating fundamental physics and spintronic applications in IV-VI compounds.