99 Aquarii (abbreviated 99 Aqr) is a star in the equatorial constellation of Aquarius. 99 Aquarii is the Flamsteed designation, although it also bears the Bayer designation b<sup>2</sup> Aquarii. It is visible to the naked eye with an apparent visual magnitude of 4.38; according to the Bortle Dark-Sky Scale this is bright enough to be seen even from city skies under ideal viewing conditions. Based upon parallax measurements, the distance to this star is around .
This is a giant star with a stellar classification of K4 III. It is a suspected variable star that apparently ranges in magnitude between 4.35 and 4.45. The measured angular diameter of this star is . At the estimated distance of Delta Ophiuchi, this yields a physical size of about 33 times the radius of the Sun. The outer atmosphere has an effective temperature of 3980 K, giving it the orange-hued glow of a cool, K-type star.
This star was a candidate member of the Ursa Major Moving Group based on the work of American astronomer Nancy Roman, but this membership is now in question.