Motility is a fundamentally important property of most members of the kinesin superfamily, but a rare subset of kinesins are also able to alter microtubule dynamics. At kinetochore-microtubule plus ends, the kinesin-8 family member Kif18A is essential to align mitotic chromosomes at the spindle equator during cell division, but how it accomplishes this function is unclear. We report here that Kif18A is a plus-end-directed motor that inhibits the polymerization dynamics of microtubule plus ends without destabilizing them, distinguishing Kif18A from the budding yeast ortholog Kip3. In interphase cells, Kif18A uses this activity to reduce the overall dynamicity of microtubule plus ends and effectively constrains the distance over which plus ends grow and shrink. Our findings suggest that kinesin-8 family members have developed biochemically distinct activities throughout evolution and have implications for how Kif18A affects kinetochore-microtubule plus-end dynamics during mitosis in animal cells.