A self-locking retractable surgical knife including a sheath member having front and rear ends, and a retractable member including a support member and a blade supported by the support member. The support member has a front end, a rear end and an elongated interior space. The retractable member is partially disposed in the sheath member and telescopically received in the sheath member for movement between a guarded position wherein the blade is guarded in the sheath member, and an exposed position wherein the blade extends out of the sheath member. The self-locking retractable further includes a biasing member disposed in the interior space of the support member and a latch member disposed in the interior space of the support member.