XTE J1739-285 is a neutron star, in the constellation Ophiuchus, situated approximately 39,000 light-years from Earth. It was first observed on 19 October 1999 by NASA's Rossi X-ray Timing Explorer satellite.
It had previously been claimed that XTE J1739-285 was the fastest-spinning celestial body yet known, with a frequency of 1122 Hz (approximately 67,320 RPM). However, a re-analysis of these data by other astronomers has been unable to reproduce this result.
XTE J1739-285 has been proposed as a possible quark star, as well as 3C 58.