The kinesin 1 family of motor proteins (KIF5A, KIF5B and KIF5C) has a well-established role in axonal transport, but its function in dendrites is less clear. This study showed that knockdown of KIF5A or KIF5B had different effects on dendritic spine morphogenesis and transport, and postsynaptic currents in cultured hippocampal neurons. Conditional Kif5b knockout in mice reduced dendritic spine number in the hippocampus, impaired excitatory transmission and caused learning and memory deficits. Thus, KIF5s show functional diversity, with a key role for KIF5B in dendrites.