Several kinesin-5 motors (kinesin-5s) exhibit bidirectional motility. The mechanism of such motility remains unknown. Bidirectional kinesin-5s share a long N-terminal nonmotor domain (NTnmd), absent in exclusively plus-end-directed kinesins. Here, we combined in vivo, in vitro, and cryo-electron microscopy (cryo-EM) studies to examine the impact of NTnmd mutations on the motor functions of the bidirectional kinesin-5, Cin8. We found that NTnmd deletion mutants exhibited cell viability and spindle localization defects. Using cryo-EM, we examined the structure of a microtubule (MT)-bound motor domain of Cin8, containing part of its NTnmd. Modeling and molecular dynamic simulations based on the cryo-EM map suggested that the NTnmd of Cin8 interacts with the C-terminal tail of β-tubulin. In vitro experiments on subtilisin-treated MTs confirmed this notion. Last, we showed that NTnmd mutants are defective in plus-end-directed motility in single-molecule and antiparallel MT sliding assays. These findings demonstrate that the NTnmd, common to bidirectional kinesin-5s, is critical for their bidirectional motility and intracellular functions.