Most adult neurons and glia originate from radial glial progenitors (RGs), a type of stem cell typically extending from the apical to the basal side of the developing cortex. Precise regulation of the choice between RG self-renewal and differentiation is critical for normal development, but the mechanisms underlying this transition remain elusive. We show that the non-canonical tubulin Tuba8, transiently expressed in cortical progenitors, drives differentiation of RGs into apical intermediate progenitors, a more restricted progenitor type lacking attachment to the basal lamina. This effect depends on the unique C-terminal sequence of Tuba8 that antagonizes tubulin tyrosination and ?2 cleavage, two post-translational modifications (PTMs) essential for RG fiber maintenance and the switch between direct and indirect neurogenesis and ultimately distinct neuronal lineage outcomes. Our work uncovers an instructive role of a developmentally regulated tubulin isotype in progenitor differentiation and provides new insights into biological functions of the cellular tubulin PTM \"code.\"