Regulated centrosome biogenesis is required for accurate cell division and for maintaining genome integrity 1 . Centrosomes consist of a centriole pair surrounded by a protein network known as pericentriolar material (PCM) 1 . PCM assembly is a tightly regulated, critical step that determines a centrosome’s size and capability 2– 4 . Here, we report a role for tubulin in regulating PCM recruitment via the conserved centrosomal protein Sas-4. Tubulin directly binds to Sas-4; together they are components of cytoplasmic complexes of centrosomal proteins 5, 6 . A Sas-4 mutant, which cannot bind tubulin, enhances centrosomal protein complex formation and has abnormally large centrosomes with excessive activity. These suggest that tubulin negatively regulates PCM recruitment. Whereas tubulin-GTP prevents Sas-4 from forming protein complexes, tubulin-GDP promotes it. Thus, tubulin’s regulation of PCM recruitment depends on its GTP/GDP-bound state. These results identify a role for tubulin in regulating PCM recruitment independent of its well-known role as a building block of microtubules 7 . Based on its guanine bound state, tubulin can act as a molecular switch in PCM recruitment.