A tunable rigidity insole with interchangeable stiffeners includes an arch pad main body and three rigid stiffeners. The arch pad main body includes a set of first to third engaging holes, and at least one or two of these three rigid stiffeners will be selected to assemble into the engaging holes on the arch pad main body. The three rigid stiffeners provide supports to lateral longitudinal, central and medial longitudinal arch portions respectively. The first, second and third rigid stiffeners have at least two engagement members for each stiffener, and the engagement members have engagement extensions with a partial length component toward another engagement extensions at the end of stiffeners thereof respectively. The engagement members engage with the arch pad main body corresponding to the engaging holes respectively. By selecting and assembling different stiffeners, the present invention can have different rigidities of arch supports to match with various foot biomechanics and gait types in order to provide different arch supports thereto properly.