Proliferating cells actively coordinate growth and cell division to ensure cell-size homeostasis; however, the underlying mechanism through which size is controlled is still poorly understood. Defect in a SUMO protease protein, SMT7, has been shown to reduce cell division number and increase cell size of the small-size mutant mat3-4, which contains a defective Chlamydomonas retinoblastoma tumor suppressor-related protein. Here we used an in vitro Chlamydomonas SUMOylation system to validate that SMT7 encodes a bona fide SUMO protease. We further demonstrate that the SUMO protease activity of SMT7 is required for supernumerous mitotic divisions of the mat3-4 cells. Additionally, we identified ribosomal protein L30 (RpL30) as a prime SMT7 target and discovered that SUMOylated RpL30 level is important to modulate cell division in mat3-4 cells. Moreover, overexpression of the translational fusion version of RpL30-SUMO4, which mimics elevated SUMOylated RpL30 protein in mat3-4, caused decrease in mitotic division and recapitulated the size-increasing phenotype of the smt7-1 mat3-4 cells. In summary, our studies reveal a novel mechanism through which a SUMO protease regulates cell division in the mat3-4 mutant of Chlamydomonas and provides yet another important example of the role that protein SUMOylation can play in regulating key cellular processes, including cell division.