The invention relates to an orthopaedic surgical instrument (1) for positioning a tibial cutting guide (2) including: at least a medial or a lateral cutting guide (3) and a vertical cutting guide (4) formed in a guide body (10); an arm or beam (11) supporting said guide body (10); a supporting rod (8) having a proximal end (18) coupled to said guide body (10) through said arm or beam (11). The arm (11) has a structure that is supported in a rotably and cantilever manner at the proximal end (18) of said rod (8); a locking mechanism (20) is provided in the proximity of said proximal end (18) for locking the arm (11) in a desired position; and a bistable mechanical switch (9) is active in said locking mechanism (20) for manually activating the locking position. In other words, the arm (11) has one end (17) hinged to the proximal end (18) of the supporting rod (8) between a terminal stopper (37) and a compressing element (31) slidably mounted in the proximity of said proximal end (18) and pushed toward the proximal direction by said bistable mechanical switch (9) of the locking mechanism (20).