Gb5 is a divergent, evolutionarily-conserved, member of the heterotrimeric G protein b subunit family that is expressed principally in brain and neuronal tissue. Among Gb isoforms, Gb5 is unique in its ability to heterodimerize with members of the R7 subfamily of the regulator of G protein signaling (RGS) proteins that contain G protein-g like (GGL) domains. Previous studies employing Gb5 knockout mice have shown that Gb5 is an essential stabilizer of GGL domain-containing RGS proteins and regulates the deactivation of retinal phototransduction and the proper functioning of retinal bipolar cells. The purpose of this study is to better understand the functions of Gb5 in the brain outside the visual system by employing molecular biology, immunohistochemistry and confocal imaging technologies. We show here that mice lacking Gb5 have a markedly abnormal neurologic phenotype that includes neurobehavioral developmental delay, wide-based gait, motor learning and coordination deficiencies, and hyperactivity. Using immunohistochemical analysis and a green fluorescent reporter of Purkinje cell maturation we show that the phenotype of Gb5-deficient mice includes, in part, delayed development of the cerebellar cortex, an abnormality that likely contributes to the neurobehavioral phenotype. Multiple neuronally-expressed genes are dysregulated in cerebellum of Gb5 KO mice.