We present proper motion measurements of Boötes III , an enigmatic stellar satellite of the Milky Way , utilizing data from the second data release of the Gaia mission . By selecting 15 radial velocity confirmed members of Boötes III , along with a likely RR Lyrae member in the vicinity , we measure an error weighted mean proper motion of ( \mu _ { \alpha } \cos { \delta } , \mu _ { \delta } ) = ( -1.14 , -0.98 ) \pm ( 0.18 , 0.20 ) mas yr ^ { -1 } . We select and present further stars that may be Boötes III members based on their combined proper motion and position in the color magnitude diagram . We caution against assigning membership to stars that are not confirmed spectroscopically , as we demonstrate that there are contaminating stars from the disrupting globular cluster NGC 5466 in the vicinity of the main body of Boötes III , but we note that our results are consistent with previous Boötes III proper motion estimates that did not include spectroscopic members . Based on the measured proper motion and other known properties of Boötes III , we derive its Galactocentric velocity and compute its orbit given canonical Milky Way potentials with halo masses of both 0.8 \times 10 ^ { 12 } M _ { \odot } and 1.6 \times 10 ^ { 12 } M _ { \odot } . These orbits robustly show that Boötes III passed within \sim 12 kpc of the Galactic center on an eccentric orbit roughly \sim 140 Myr ago . Additionally , the proper motion of Boötes III is in excellent agreement with predictions for the retrograde motion of the coincident Styx stellar stream . Given this , along with the small pericenter and metallicity spread of Boötes III itself , we suggest that it is a disrupting dwarf galaxy giving rise to the Styx stellar stream .