Watermarking is to embed a structure called a watermark into the target data such as images. The watermark can be used, for example, in order to secure the copyright and detect tampering. This paper presents a new robust watermarking method that adds a watermark into a 3D polygonal mesh in the spectral domain. In this algorithm, a shape of a 3D polygonal model is regarded as a sequence of vertices called a vertex series. The spectrum of the vertex series is computed using the singular spectrum analysis (SSA) for the trajectory matrix derived from the vertex series. Watermarks embedded by this method are resistant to similarity transformations and random noises.