There has been renewed interest in the use of polynomial and homogeneous polynomial Lyapunov functions for addressing stability and performance issues in systems and control theory. However, no condition is yet available to establish the convexity of these functions and the convexity of their sublevel sets, properties that can be useful in many applications. In this paper, new conditions for establishing these convexity properties are proposed, which can be handled through convex linear matrix inequalities optimizations by means of sum-of-squares relaxations. to