S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.

S-types are moderately bright (with an albedo of 0.10 to 0.22) and consist mainly of iron- and magnesium-silicates. They are dominant in the inner asteroid belt within 2.2 AU, common in the central belt within about 3 AU, but become rare farther out. The largest is 15 Eunomia (about 330 km wide across its longest dimension), with the next largest members by diameter being 3 Juno, 29 Amphitrite, 532 Herculina and 7 Iris. These largest S-types are visible in 10x50 binoculars at most oppositions; the brightest, 7 Iris, can occasionally become brighter than +7.0, which is a higher magnitude than any asteroid except the unusually reflective 4 Vesta.

Their spectrum has a moderately steep slope at wavelengths shorter than 0.7 µm, and has moderate to weak absorption features around 1 µm and 2 µm. The 1 µm absorption is indicative of the presence of silicates (stony minerals). Often there is also a broad but shallow absorption feature centered near 0.63 µm. The composition of these asteroids is similar to a variety of stony meteorites which share similar spectral characteristics.

25143 Itokawa ( /ˌiːtoʊˈkɑːwə/; Japanese: イトカワ [itokawa]) is an Apollo and Mars-crosser asteroid. It was the first asteroid to be the target of a sample return mission, the Japanese space probe Hayabusa.

The asteroid was discovered in 1998 by the LINEAR project and was given the provisional designation 1998 SF₃₆. In August 2003, it was officially named after Hideo Itokawa, a Japanese rocket scientist.

Itokawa is an S-type asteroid. Radar imaging by Goldstone in 2001 observed an ellipsoid 630 ± 60 m long and 250 ± 30 m wide.

The Hayabusa mission confirmed these findings and also suggested that Itokawa may be a contact binary formed by two or more smaller asteroids that have gravitated toward each other and stuck together. The Hayabusa images show a surprising lack of impact craters and a very rough surface studded with boulders, described by the mission team as a 'rubble pile'. Furthermore, the density of the asteroid is too low for it to be made from solid rock. This would mean that Itokawa is not a monolith but rather a ‘rubble pile’ formed from fragments that have cohered over time.