AbstractThe blue compact dwarf galaxy NGC 5253 hosts a very young starburst containing twin nuclear star clusters. Calzetti et al. (2015) find that the two clusters have an age of 1 Myr, in contradiction to the age of 3–5 Myr inferred from the presence of Wolf-Rayet (W-R) spectral features. We use Hubble Space Telescope (HST) far-ultraviolet (FUV) and ground-based optical spectra to show that the cluster stellar features arise from very massive stars (VMS), with masses greater than 100 M_⊙, at an age of 1–2 Myr. We discuss the implications of this and show that the very high ionizing flux can only be explained by VMS. We further discuss our findings in the context of VMS contributing to He ii λ1640 emission in high redshift galaxies, and emphasize that population synthesis models with upper mass cut-offs greater than 100 M_⊙ are crucial for future studies of young massive clusters.