Presolar grains are interstellar solid matter in the form of tiny solid grains that originated at a time before the Sun was formed. Presolar grains formed within outflowing and cooling gases from earlier presolar stars. The study of presolar grains is typically considered part of the field of cosmochemistry and meteoritics.

The stellar nucleosynthesis that took place within each presolar star gives to each granule an isotopic composition unique to that parent star, which differs from the isotopic composition of the Solar System's matter as well as from the galactic average. These isotopic signatures often fingerprint very specific astrophysical nuclear processes that took place within the parent star or formation event and prove their presolar origin.

Presolar grains are individual solid grains which condensed around distant stars or as part of novae, and potentially supernovae outflows, which were accreted in the early solar nebula and remain in relatively unaltered chondritic meteorites. As they were accreted before the formation of the Solar System, they must be presolar. Presolar grains also exist in the interstellar medium. Researchers occasionally use the term stardust to refer to presolar grains, particularly in science communication, though the term is sometimes used interchangeably in the scientific literature.

In the 1960s, the noble gases neon and xenon were discovered to have unusual isotopic ratios in primitive meteorites; their origin and the type of matter that contained them was a mystery. These discoveries were made by vaporizing a bulk sample of a meteorite within a mass spectrometer, in order to count the relative abundance of the isotopes of the very small amount of noble gases trapped as inclusions. During the 1970s similar experiments discovered more components of trapped xenon isotopes. Competing speculations about the origins of the xenon isotopic components were advanced, all within the existing paradigm that the variations were created by processes within an initially homogeneous solar gas cloud.

A new theoretical framework for interpretation was advanced during the 1970s when Donald D. Clayton rejected the popular belief among meteoriticists that the Solar System began as a uniform hot gas. Instead he predicted that unusual but predictable isotopic compositions would be found within thermally condensed interstellar grains that had condensed during mass loss from stars of differing types. He argued that such grains exist throughout the interstellar medium. Clayton's first papers using that idea in 1975 pictured an interstellar medium populated with supernova grains that are rich in the radiogenic isotopes of Ne and Xe that had defined the extinct radioactivities. Clayton defined several types of presolar grains likely to be discovered: stardust from red giant stars, sunocons (acronym from SUperNOva CONdensates) from supernovae, nebcons from nebular condensation by accretion of cold cloud gaseous atoms and molecules, and novacons from nova condensation. Despite vigorous and continuous active development of this picture, Clayton's suggestions lay unsupported by others for a decade until such grains were discovered within meteorites.

The first unambiguous consequence of the existence of presolar grains within meteorites came from the laboratory of Edward Anders in Chicago, who found using traditional mass spectrometry that the xenon isotopic abundances contained within an acid-insoluble carbonaceous residue that remained after the meteorite bulk had been dissolved in acids matched almost exactly the predictions for isotopic xenon in red giant dust condensate. It then seemed certain that presolar grains were contained within Anders' acid-insoluble residue. Finding the actual presolar grains and documenting them was a much harder challenge that required locating the grains and showing that their isotopes matched those within the red-giant star. There followed a decade of intense experimental searching in the attempt to isolate individual grains of those xenon carriers. But what was really needed to discover presolar grains was a new type of mass spectrometer that could measure the smaller number of atoms in a single grain. Sputtering ion probes were pursued by several laboratories in the attempt to demonstrate such an instrument. But the contemporary ion probes needed to be technologically much better.

In 1987 diamond grains and silicon carbide grains were found to exist abundantly in those same acid-insoluble residues and also to contain large concentrations of noble gases. Significant isotopic anomalies were in turn measured by improvements in secondary ion mass spectrometry (SIMS) within the structural chemical elements of these grains. Improved SIMS experiments showed that the silicon isotopes within each SiC grain did not have solar isotopic ratios but rather those expected in certain red-giant stars. The finding of presolar is therefore dated 1987. To measure the isotopic abundance ratios of the structural elements (e.g. silicon in an SiC grain) in microscopic presolar grains had required two difficult technological and scientific steps: 1) locating micron-sized presolar grains within the meteorite's overwhelming mass; 2) development of SIMS technology to a sufficiently high level to measure isotopic abundance ratios within micron-sized grains. Ernst Zinner became an important leader in SIMS applications to microscopic grains.

In January 2020, analysis of the Murchison meteorite concluded that out of 40 presolar silicon carbide grains examined, one had formed 3 ± 2 billion years before Earth's 4.6 billion year-old sun. This would make some of the grains the oldest solid material ever discovered on Earth.