In physics, a fifth force is a hypothetical fundamental interaction (also known as fundamental force) beyond the four known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculative theories have proposed a fifth force to explain various anomalous observations that do not fit existing theories. The specific characteristics of a putative fifth force depend on which hypothesis is being advanced. No evidence to support these models has been found.

The term is also used as "the Fifth force" when referring to a specific theory advanced by Ephraim Fischbach in 1971 to explain experimental deviations in the theory of gravity. Later analysis failed to reproduce those deviations.

The term fifth force originates in a 1986 paper by Ephraim Fischbach et al. who reanalyzed the data from the Eötvös experiment of Loránd Eötvös from earlier in the century; the reanalysis found a distance dependence to gravity that deviates from the inverse square law. The reanalysis was sparked by theoretical work in 1971 by Fujii proposing a model that changes distance dependence with a Yukawa potential-like term: $${\displaystyle V(r)=-G_{\infty }{\frac {m_{i}m_{j}}{r_{ij}}}(1+\alpha e^{-r/\lambda })}$$ The parameter ${\displaystyle \alpha }$ characterizes the strength and ${\displaystyle \lambda }$ the range of the interaction. Fischbach's paper found a strength around 1% of gravity and a range of a few hundred meters. The effect of this potential can be described equivalently as exchange of vector and/or scalar bosons, that is a predicting as yet undetected new particles. However, many subsequent attempts to reproduce the deviations have failed.

Theoretical proposals for a fifth force are driven by inconsistencies between the existing models of general relativity and quantum field theory, and also between the hierarchy problem and the cosmological constant problem. Both issues suggest the possibility of corrections to the gravitational potential around 100 μm.

The accelerating expansion of the universe has been attributed to a form of energy called dark energy. Some physicists speculate that a form of dark energy called quintessence could be a fifth force.

There are at least three kinds of searches that can be undertaken, which depend on the kind of force being considered, and its range.

One way to search for a fifth force is with tests of the strong equivalence principle, one of the most powerful tests of general relativity, also known as Einstein's theory of gravity. Alternative theories of gravity, such as Brans–Dicke theory, postulate a fifth force — possibly one with infinite range. This is because gravitational interactions, in theories other than general relativity, have degrees of freedom other than the "metric", which dictates the curvature of space, and different kinds of degrees of freedom produce different effects. For example, a scalar field cannot produce the bending of light rays.

The fifth force would manifest itself in an effect on solar system orbits, called the Nordtvedt effect. This is tested with Lunar Laser Ranging experiment and very-long-baseline interferometry.