We report on the design, calibration and in-flight performance of a sun sensor, which is used to determine the attitude of a balloon-borne telescope. The device uses a position-sensitive detector (PSD) in a pinhole camera. By determining the position of the image of the Sun on the PSD, the orientation of the sun sensor and the boresight of the telescope relative to the Sun can be determined. The pinhole sun sensor (PSS) was first flown in the December 2010 flight of the Balloon-borne Large Aperture Submillimeter Telescope with Polarization (BLAST-Pol). In flight the PSS achieved an accuracy (combined azimuth and elevation) of about 0.18°. The accuracy could be improved by increasing the distance between the pinhole and the PSD, but the field-of-view of the PSS would be reduced.