We determine the frequency of the ultranarrow Sr87S01-P23 transition by spectroscopy of an ultracold gas. This transition is referenced to four molecular iodine lines that are observed by Doppler-free saturation spectroscopy of hot iodine vapor. The frequency differences between the Sr and the I2 transitions are measured with an uncertainty of 0.5MHz. The absolute frequency of the Sr87S01-P23 (F′=7/2) transition is 446648775(30)MHz and limited in accuracy by the iodine reference. This work prepares the use of the Sr S01-P23 transition for quantum simulation and computation.