This paper proposes a low-profile and highly efficient endfire radiating travelling-wave antenna based on spoof surface plasmon polaritons (SSPPs) transmission line. The aperture is approximately $0.32lambda_0times0.01lambda_0$ where $lambda_0$ is the space wavelength at the operational frequency 8 GHz. This antenna provides an endfire radiation beam within 7.5-8.5 GHz. The maximum gain and total efficiency reaches 9.2 dBi and $96%$, respectively. In addition to the endfire operation, it also provides a beam scanning functionality within 9-12 GHz. Measurement results are finally given to validate the proposed SSPPs antenna.