In this paper, we consider a downlink scenario in which a base station (BS) communicates with user equipment (UE) that move with high speed. Orthogonal time frequency space (OTFS) modulation is used to overcome the effects of high Doppler shifts inherent with high-mobility scenarios. To increase spectral efficiency, power-domain non-orthogonal multiple access (NOMA) is combined with OTFS to form an OTFS-NOMA system. The K-means clustering algorithm is deployed which partitions the coverage area of the BS into clusters such that the UEs within a cluster have a shorter distance to the respective cluster center than to the centers of the other clusters. The cluster centers act as simultaneous multiple users on which basis power-domain NOMA selects the power allocation coefficients to reduce the mutual interference among UEs. OTFS can control the resolution of the delay-Doppler domain to more accurately compensate for propagation delays and Doppler shifts. The performance of this OTFS-NOMA system with K-means clustering of the UEs is assessed in terms of sum rate. The numerical results illustrate that the successive interference cancellation of power-domain NOMA significantly improves sum rate. Additional improvements are obtained by choosing a sufficiently large size of the delay-Doppler grid of OTFS which effectively copes with the high Doppler shifts of the considered high-mobility scenario. © 2023 IEEE.