The authors address a key issue in unsaturated soil mechanics concerning the correct use of the concept of matrix suction: is matrix suction a stress variable or a stress state variable? Aim. To determine the correct approach to describe matrix suction and its effect on the behavior of unsaturated soils. Materials and methods. Generally accepted principle of mechanical equilibrium and the concept of representative elementary volume for multiphase porous media including air, water and solid particles. The authors generalized the commonly accepted physical and logical reasoning to analyze matrix suction under typical air–water–solid conditions. Results. The study revealed that matrix suction is not a stress variable at the typical air–water–solid level. Instead, it should be considered as a stress state variable. It is also found that when considering the matrix suction as a stress state variable, there is a relationship between it and normal stress when used simultaneously to describe the stress state of unsaturated soils. The results obtained have important implications for the concept, theorizing, and application of unsaturated soil mechanics. Understanding that matrix suction is a stress state variable and is related to normal stress allows for more accurate modeling of unsaturated soil behavior and the development of more robust engineering solutions. This study contributes to the advancement of soil mechanics knowledge and provides new directions for future research.