140 Siwa is a large and dark main belt asteroid. It has a composition of a P (or possibly C-type) asteroid.

It was discovered by J. Palisa on October 13, 1874 and named after Siwa, the Slavic goddess of fertility.

Siwa has a very flat lightcurve, indicating a spherical body.

The Rosetta comet probe was to visit Siwa on its way to comet 46P/Wirtanen in July, 2008. However, the mission was rerouted to comet 67P/Churyumov-Gerasimenko and the flyby had to be abandoned.