An exhaust purification system for an internal combustion engine is provided that can steadily maintain a NOx purification rate of a selective reduction catalyst to be high without allowing the fuel economy or marketability to deteriorate. The exhaust purification system includes a NO2—NOx ratio adjustment mechanism that causes a NO2—NOx ratio to change; and a NO2—NOx ratio perturbation controller that executes NO2—NOx ratio perturbation control so that a NO2 balance of the selective reduction catalyst in a predetermined time period, with NO2 adsorption being positive and NO2 release being negative, is 0. Herein, NO2—NOx ratio perturbation control is defined as control that alternately executes NO2 increase control to cause the NO2—NOx ratio to be greater than a reference value near 0.5, and NO2 decrease control to cause the NO2—NOx ratio to be less than the reference value.