The present invention discloses a cutter for trimming a tread of a tire. The cutter includes a cutting plate which has a plurality of cutting slits sequentially formed at a lower end thereof to cut over-flow rubber formed on the tread of a tire; and a handle which is coupled to the cutting plate, and is configured for a user to hold to cut the over-flow rubber using the cutting plate. The cutting plate has a plurality of grooves formed on a rear surface thereof to vertically guide and discharge the over-flow rubber cut by the cutting slits. According to the present invention, it is possible to prevent a reduction in a lifespan due to a decrease in hardness of a cutting plate caused by friction heat generated in a process of using the cutter for trimming the tread of a tire, and prevent over-flow rubber discharged in a process of trimming the tire from scattering.