A leadless cardiac pacemaker having a radial fixation mechanism is provided. The cardiac pacemaker can include fixation mechanism separate from a pacing electrode and having a diameter equal to or less than the outer diameter of the pacemaker. The fixation mechanism can allow the pacemaker to be inserted into tissue with less than 2 rotations of the pacemaker to place the pacing electrode in contact with the tissue. In some embodiments, the fixation mechanism can comprise a plurality of hooks or protrusions positioned near a distal portion of the pacemaker. The fixation mechanism(s) can be configured to penetrate the endocardium of the patient and reside mostly within the myocardium. Methods of delivering the leadless cardiac pacemaker into the heart are also provided.