In physics, tachyon condensation is a process in which a tachyonic field—usually a scalar field—of a negative squared mass acquires a vacuum expectation value and reaches the minimum of the potential energy. While the field is tachyonic (and unstable) near the original point—the maximum of the potential—it gets a non-negative mass (and becomes stable) near the minimum.

The appearance of tachyons is a potentially lethal problem for any theory: the notion of an imaginary mass is troubling, and many proposed tachyon models involve violating causality. The understanding of tachyon condensation seems to be the only way to make such a theory meaningful. Tachyon condensation drives the physical system to a stable state where no physical tachyons exist.

The Higgs mechanism that breaks the electroweak symmetry may be understood as a simple example of tachyon condensation. In the late 1990s, the Indian string theorist Ashoke Sen conjectured that the tachyons carried by open strings attached to D-branes in string theory reflect the instability of the D-branes with respect to their complete annihilation. The total energy carried by these tachyons has been calculated in string field theory; it agrees with the total energy of the D-branes, and all other tests have confirmed Sen's conjecture as well. Tachyons therefore experienced a comeback in the early 2000s.

The character of closed string tachyons is more subtle, and the first steps towards our understanding of their fate have been made by Adams, Polchinski, and Silverstein in the case of twisted closed string tachyons. The fate of the closed string tachyon in the 26-dimensional bosonic string theory remains unknown.