R Leonis is a Mira star, 390 light-years away in the constellation Leo, that is one the brightest and easiest to observe variable stars in the sky. With a mean visual magnitude range of 5.8 to 10.0, it can be followed throughout its average 312-day cycle with a small telescope, and for much of that time with binoculars. Discovered by J. A. Koch of Danzig in 1782, R Leonis was the fifth variable star and fourth long-period variable to be found after Mira itself, Chi Cygni, and R Hydrae. It lies about 5° west of Regulus in the direction of Xi Leonis.
As a result of data collected by the Hubble Space Telescope, it is one of handful of Mira stars known to be somewhat egg-shaped, with an apparent diameter of 70 × 78 milliarcseconds. This asymmetry is thought to involve the star's extended atmosphere, but its cause is not properly understood. It may be a result of non-radial pulsations (the star not pulsating equally in all directions) or it may be an optical illusion as a result of large dark spots on the star, perhaps caused by giant convection cells. Similar indications of an oblong shape have been found in the case of Mira itself, R Cassiopeia, and W Hydrae.