ABSTRACT We present the first comprehensive thermal and rotational analysis of the second most distant trans-Neptunian object (TNOs) (225088) 2007 OR10. We combined optical light curves provided by the Kepler Space Telescope-K2 extended mission and thermal infrared data provided by the Herschel Space Observatory. We found that (225088) 2007 OR10 is likely to be larger and darker than derived by earlier studies: we obtained a diameter of d = 1535 − 225 + 75 km which places (225088) 2007 OR10 in the biggest top three TNOs. The corresponding visual geometric albedo is p V = 0.089 − 0.009 + 0.031 . The light-curve analysis revealed a slow rotation rate of Prot = 44.81 0.37 hr, superseded by very few objects. The most likely light-curve solution is double-peaked with a slight asymmetry; however, we cannot safely rule out the possibility of having a rotation period of Prot = 22.40 0.18 hr, which corresponds to a single-peaked solution. Due to the size and slow rotation, the shape of the object should be a MacLaurin ellipsoid, so the light variation should be caused by surface inhomogeneities. Its newly derived larger diameter also implies larger surface gravity and a more likely retention of volatiles-CH4, CO, and N2-on the surface.