66652 Borasisi, or as a binary (66652) Borasisi–Pabu (provisional designation 1999 RZ₂₅₃), is a binary classical Kuiper belt object. It was discovered in September 1999 by Chad Trujillo, Jane X. Luu and David C. Jewitt and identified as a binary on 23 August 2003 by K. Noll and colleagues using the Hubble Space Telescope.

In 2003 it was discovered that Borasisi is a binary with the components of comparable size (about 100–130 km) orbiting the barycentre on a moderately elliptical orbit. The total system mass is about 3.4 × 10¹⁸ kg.

The companion (66652) Borasisi I, named Pabu, orbits its primary in 46.2888 ± 0.0018 days on an orbit with a semi-major axis of 4528 ± 12 km and an eccentricity 0.4700 ± 0.0018. The orbit is inclined with respect to the observer by about 54° meaning that is about 35° from the pole-on position.

The surface of both components of the Borasisi–Pabu system is very red.

Borasisi is named after a fictional creation deity taken from the novel Cat's Cradle by Kurt Vonnegut. In the book, Borasisi is the Sun and Pabu is the name of the Moon:

Around 2005, Borasisi was considered as a target for the proposed New Horizons 2 after a Triton/Neptune flyby.