Ordinary materials have positive Poisson's ratios (PRs), and the currently known mechanical metamaterials possess negative PRs both under tension and compression. Herein, two types of novel cellular structure units with tension/compression‐dependent positive/negative PRs are demonstrated. For type I structure, the unit exhibits a positive PR during tension and a negative PR during compression, while for type II structure, the unit shows a negative PR during tension and a positive PR during compression. The deformation behaviors of the new metamaterials are first predicted by a simple analytical model and finite‐element method calculations, and the predicted results are then confirmed experimentally. Numerical and experimental studies show that for the plane angle α<90° , the unit of the new metamaterial switches its PR under compression and exhibits a discontinuous change in the PR. The deformation law does not depend on either the Young's modulus or the hardness of the material. The geometrically determined deformation behaviors of the proposed metamaterials are observed in 2D and 3D cellular structures and are proven to be robust through numerical and experimental studies. The proposed mechanical metamaterials can be applied as new deformation‐protected materials in energy‐absorbing devices, soft robotics, and tissue engineering scaffolds and as thermal deformation materials.