M-type (metallic-type, a.k.a. M-class) asteroids are a spectral class of asteroids which appear to contain higher concentrations of metal phases (e.g. iron-nickel) than other asteroid classes, and are widely thought to be the source of iron meteorites.

Asteroids are classified as M-type based upon their generally featureless and flat to red-sloped absorption spectra in the visible to near-infrared and their moderate optical albedo. Along with the spectrally similar E-type and P-type asteroids (both categories E and P were formerly type-M in older systems), they are included in the larger X-type asteroid group and are distinguishable only by optical albedo:

Although widely assumed to be metal-rich (the reason for use of "M" in the classification), the evidence for a high metal content in the M-type asteroids is only indirect, though highly plausible. Their spectra are similar to those of iron meteorites and enstatite chondrites, and radar observations have shown that their radar albedos are much higher than other asteroid classes, consistent with the presence of higher density compositions like iron-nickel. Nearly all of the M-types have radar albedos at least twice as high as the more common S- and C-type, and roughly one-third have radar albedos ~3× higher.

High resolution spectra of the M-type have sometimes shown subtle features longward of 0.75 μm and shortward of 0.55 μm. The presence of silicates is evident in many, and a significant fraction show evidence of absorption features at 3 μm, attributed to hydrated silicates. The presence of silicates, and especially hydrated silicates, is at odds with the traditional interpretation of M-types as remnant iron cores.

The bulk density of an asteroid provides clues about its composition and meteoritic analogs. For the M-types, the proposed analogs have bulk densities that range from ~3 g/cm³ for some types of carbonaceous chondrites up to nearly 8 g/cm³ for the iron-nickel present in iron-meteorites. Given the bulk density of an asteroid and the density of the materials that make it up (a.k.a. particle or grain density), one can calculate its porosity and infer something of its internal structure; for example, whether an object is coherent, a rubble pile, or something in-between.

To calculate the bulk density of an asteroid requires an accurate estimate of its mass and volume; both of these are difficult to obtain given their small size relative to other Solar System objects. In the case of the larger asteroids, one can estimate mass by observing how their gravitational field affects other objects, including other asteroids and orbiting or flyby spacecraft. If an asteroid possesses one or more moons, one can use their collective orbital parameters (e.g. orbital period, semimajor axis) to estimate the masses of the ensemble, for example in the two-body problem.

To estimate an asteroid's volume requires, at a minimum, an estimate of an asteroid's diameter. In most cases, these are estimated from the visual albedo (brightness) of the asteroid, chord-lengths during occultations, or their thermal emissions (e.g. IRAS mission). In a few cases, astronomers have managed to develop three-dimensional shape models using a variety of techniques (cf. 16 Psyche or 216 Kleopatra for examples) or, in a few lucky instances, from spacecraft imaging (cf. 162173 Ryugu).

Of these, mass measurements made via spacecraft deflection or the orbits of moons are considered the most reliable. Ephemeris estimates are based on the subtle gravitational pull of other objects on that asteroid, or vice versa, and are considered less reliable. The exception to this caveat may be Psyche, as it is the most massive M-type asteroid and has numerous mass estimates. Size estimates based on shape models (usually derived from adaptive optics, occultations, and radar imaging) are the most reliable. Direct spacecraft imaging (Lutetia) is also quite reliable. Sizes based on indirect methods like thermal IR (e.g. IRAS) and radar echoes are less reliable.