This paper presents a stiffness analysis of a four-leg tensegrity mechanism. Tensegrity mechanisms are known for their simple and lightweight nature, making them suitable for a variety of applications, particularly in robotics. The study focuses on the ability of the mechanism to vary its stiffness through antagonistic actuation and maintain stability under specific actuation conditions. A reduced static formulation based on the Newton-Euler equations is used to model the stiffness matrix in generalized coordinates. The stiffness matrix is determined using numerical approximation methods according to specific conditions on the cable tensions. The results highlight the mechanism’s ability to adjust stiffness and ensure stability, providing valuable insights for the design and optimization of tensegrity structures for robotics.