21795 Masi (provisional designation 1999 SN₉) is a Nysian asteroid from the inner regions of the asteroid belt, approximately 3 kilometers (2 miles) in diameter. It was discovered on 29 September 1999, by Italian amateur astronomer Franco Mallia at the Campo Catino Astronomical Observatory in Lazio, Italy.^[1] The likely S-type asteroid has a rotation period of 13.86 hours.^[4] It was named for Italian astronomer Gianluca Masi.^[1]

Masi is member of the Nysa family (405),^[3] located in the Nysa–Polana complex and one of the largest asteroid families of the asteroid belt, named after 44 Nysa.^[9] It orbits the Sun in the inner main-belt at a distance of 1.9–2.8 AU once every 3 years and 8 months (1,342 days; semi-major axis of 2.38 AU). Its orbit has an eccentricity of 0.19 and an inclination of 2° with respect to the ecliptic.^[2] The body's observation arc begins with a precovery published by the Digitized Sky Survey and taken at the Palomar Observatory in April 1954, more than 45 years prior to its official discovery observation at Campo Catino.^[1]

Masi is an assumed, stony S-type asteroid,^[4] which is also the overall spectral type for members of the Nysa family of asteroids.^[9]: 23

This minor planet was named after Italian astrophysicist and astronomer, Gianluca Masi (born 1972), a researcher and discoverer of minor planets and variable stars, who became an avid amateur astronomer when he was 8 years old.^[1] The official naming citation was published by the Minor Planet Center on 9 May 2001 (M.P.C. 42679).^[10]