Soft matter or soft condensed matter is a type of matter that can be deformed or structurally altered by thermal or mechanical stress which is of similar magnitude to thermal fluctuations.

The science of soft matter is a subfield of condensed matter physics. Soft materials include liquids, colloids, polymers, foams, gels, granular materials, liquid crystals, flesh, and a number of biomaterials. These materials share an important common feature in that predominant physical behaviors occur at an energy scale comparable with room temperature thermal energy (of order of kT), and that entropy is considered the dominant factor. At these temperatures, quantum aspects are generally unimportant. When soft materials interact favorably with surfaces, they become squashed without an external compressive force.

Pierre-Gilles de Gennes, who has been called the "founding father of soft matter," received the Nobel Prize in Physics in 1991 for discovering that methods developed for studying order phenomena in simple systems can be generalized to the more complex cases found in soft matter, in particular, to the behaviors of liquid crystals and polymers.

The current understanding of soft matter grew from Albert Einstein's work on Brownian motion, understanding that a particle suspended in a fluid must have a similar thermal energy to the fluid itself (of order of kT). This work built on established research into systems that would now be considered colloids.

The crystalline optical properties of liquid crystals and their ability to flow were first described by Friedrich Reinitzer in 1888, and further characterized by Otto Lehmann in 1889. The experimental setup that Lehmann used to investigate the two melting points of cholesteryl benzoate are still used in the research of liquid crystals as of about 2019.

In 1920, Hermann Staudinger, recipient of the 1953 Nobel Prize in Chemistry, was the first person to suggest that polymers are formed through covalent bonds that link smaller molecules together. The idea of a macromolecule was unheard of at the time, with the scientific consensus being that the recorded high molecular weights of compounds like natural rubber were instead due to particle aggregation.

The use of hydrogel in the biomedical field was pioneered in 1960 by Drahoslav Lím and Otto Wichterle. Together, they postulated that the chemical stability, ease of deformation, and permeability of certain polymer networks in aqueous environments would have a significant impact on medicine, and were the inventors of the soft contact lens.

These seemingly separate fields were dramatically influenced and brought together by Pierre-Gilles de Gennes. The work of de Gennes across different forms of soft matter was key to understanding its universality, where material properties are not based on the chemistry of the underlying structure, more so on the mesoscopic structures the underlying chemistry creates. He extended the understanding of phase changes in liquid crystals, introduced the idea of reptation regarding the relaxation of polymer systems, and successfully mapped polymer behavior to that of the Ising model.