The invention relates to an exhaust nozzle for a bypass turbomachine, the nozzle comprising a central body, a primary cowl surrounding the central body to define a primary channel, and a secondary cowl surrounding the primary cowl to define a secondary annular channel, the secondary cowl comprising a stationary portion and a movable portion disposed to extend the stationary portion and capable of moving longitudinally upstream and downstream relative to the stationary portion and relative to the primary cowl so as to vary the exhaust section and/or the throat section of the nozzle, the stationary portion of the secondary cowl presenting a plurality of spaced-apart repetitive patterns disposed extending its trailing edge, and the movable portion of the secondary cowl including in its outside surface a plurality of indentations of shapes complementary to the patterns of the stationary portion.