A subdwarf, sometimes denoted by "sd", is a star with luminosity class VI under the Yerkes spectral classification system. They are defined as stars with luminosity 1.5 to 2 magnitudes lower than that of main-sequence stars of the same spectral type. On a Hertzsprung–Russell diagram subdwarfs appear to lie below the main sequence.

The term "subdwarf" was coined by Gerard Kuiper in 1939, to refer to a series of stars with anomalous spectra that were previously labeled as "intermediate white dwarfs".

Since Kuiper coined the term, the subdwarf type has been extended to lower-mass stars than were known at the time. Astronomers have also discovered an entirely different group of blue-white subdwarfs, making two distinct categories:

Like ordinary main-sequence stars, cool subdwarfs (of spectral types G to M) produce their energy from hydrogen fusion. The explanation of their underluminosity lies in their low metallicity: These stars are not enriched in elements heavier than helium. The lower metallicity decreases the opacity of their outer layers and decreases the radiation pressure, resulting in a smaller, hotter star for a given mass. This lower opacity also allows them to emit a higher percentage of ultraviolet light for the same spectral type relative to a Population I star, a feature known as ultraviolet excess. Usually members of the Milky Way's halo, they frequently have high space velocities relative to the Sun.

Cool subdwarfs of spectral type L and T exist, such as ULAS J131610.28+075553.0 with spectral type sdT6.5.

Subclasses of cool subdwarfs are as following:

The low metallicity of subdwarfs is coupled with their old age. The early universe had a low content of elements heavier than helium and formed stars and brown dwarfs with lower metallicity. Only later supernovae, planetary nebulae and neutron star mergers enriched the universe with heavier elements. The old subdwarfs belong therefore often to the older structures in our Milky Way, mainly the thick disk and the galactic halo. Objects in the thick disk or the halo have a high space velocity compared to the Sun, which belongs to the younger thin disk. A high proper motion can be used to discover subdwarfs. Additionally the subdwarfs have spectral features that make them different from subdwarfs with solar metallicity. All subdwarfs share the suppression of the near-infrared spectrum, mainly the H-band and K-band. The low metallicity increase the collision induced absorption of hydrogen, causing this suppressed near-infrared spectrum. This is seen as blue infrared colors compared to brown dwarfs with solar metallicity. The low metallicity also change other absorption features, such as deeper CaH and TiO bands at 0.7 μm in L-subdwarfs, a weaker VO band at 0.8 μm in early L-subdwarfs and stronger FeH band at 0.99 μm for mid- to late L-subdwarfs. 2MASS J0532+8246 was discovered in 2003 as the first L-type subdwarf, which was later re-classified as an extreme subdwarf. The L-type subdwarfs have subtypes similar to M-type subdwarfs: The subtypes subdwarf (sd), extreme subdwarfs (esd) and ultra subdwarfs (usd), which are defined by their decreasing metallicity, compared to solar metallicity, which is defined on a logarithmic scale:

For T-type subdwarfs only a small sample of subdwarfs and extreme subdwarfs is known.