42 Isis is a large main-belt asteroid, measuring 100.2 km in diameter with a stony (S-type) composition.

The asteroid's name was chosen by Manuel John Johnson, director of the Radcliffe Observatory in Oxford.

The light curve inversion technique, when applied to photometric observations of this asteroid, show multiple local irregularities.

The overall shape displays little elongation, with a ratio between the major and minor axes equal to 1.1.

[4] The spectrum of 42 Isis reveals the strong presence of the mineral olivine, a relative rarity in the asteroid belt.