Subject motion is still the major source of data quality degradation in functional magnetic resonance imaging (fMRI) studies. Established methods correct motion between successive repetitions based on the acquired imaging volumes either retrospectively or prospectively. A fast, highly accurate, and prospective real-time correction method for fMRI using external optical motion tracking has been implemented. The head position is determined by means of an optical stereoscopic tracking system. The method corrects motion during the acquisition of an fMRI time series on a slice-by-slice basis by continuously updating the imaging volume position to follow the motion of the head. This method allows the measurement of fMRI data in the presence of significant motion during the acquisition of a single volume. Even without intentional motion, fMRI signal stability is maintained and higher sensitivity to detect activation is achieved without reducing specificity. With significant motion, only the proposed approach allowed detection of brain activation. The results show that the new method is superior to image-based correction methods, which fail in the case of fast or excessive motion.