"DEVICE INTRODUCED IN HEADRESTS TO BE ADAPTED TO BACKRESTS OF SEATS AND/OR EASY CHAIRS", comprising a pair of parts (10, 20, 20a, 20b) articulated among themselves by their edges (11, 21), mentioned parts (10, 20, 20a, 20b) obtained in a single mold by blow process, each part (10, 20, 20a, 20b) presenting in its external face (12, 22) a convex contour surface substantially trapezoidal with round side edges while each part (10, 20, 20a, 20b) is internally provided with recesses, shoulders, chute-like recesses and convolutes (13, 14, 23, 24, 25, 26, 27, 28, 29) adequately disposed, assuring male/female fitting among parts (10, 20, 20a, 20b) and a metallic structure (30) that supports the headrest in its assembled condition on the upper edge of the backrest (E) of the seat or easy chair.