A wideband and high-gain planar endfire antenna based on spoof surface plasmon polaritons (SSPPs) at millimeter-wave (MMW) band is presented. First, a unilaterally corrugated surface antenna is demonstrated to radiate endfire beam tilted up from the flat plane of the corrugated metal teeth due to the effect of finite ground plane. Then, a SSPPs endfire antenna is designed with two symmetrically corrugated surfaces on the double sides of a metal strip. To excite the desired beam, the feeding network is comprised of a power divider and two L-shaped slots, so as to simultaneously produce oppositely tangential electric fields and identically normal electric fields. Moreover, the two equivalent identically tangential magnetic current sources above the corrugated surfaces are excited by flared feeding slot transitions. As a result, a synthetic endfire beam with zero-tilted angle is achieved. In final, such an antenna, operating in a range from 30 to 50 GHz with 50% fractional bandwidth, is designed with maximum gain of 16 dBi, as well confirmed in the experiment. The proposed SSPPs endfire antenna holds a very low profile and it is promising for application in MMW communication systems.