In physics, a transverse wave is a wave that oscillates perpendicularly to the direction of the wave's advance. In contrast, a longitudinal wave travels in the direction of its oscillations. All waves move energy from place to place without transporting the matter in the transmission medium if there is one. Electromagnetic waves are transverse without requiring a medium. The designation “transverse” indicates the direction of the wave is perpendicular to the displacement of the particles of the medium through which it passes, or in the case of EM waves, the oscillation is perpendicular to the direction of the wave.

A simple example is given by the waves that can be created on a horizontal length of string by anchoring one end and moving the other end up and down. Another example is the waves that are created on the membrane of a drum. The waves propagate in directions that are parallel to the membrane plane, but each point in the membrane itself gets displaced up and down, perpendicular to that plane. Light is another example of a transverse wave, where the oscillations are the electric and magnetic fields, which point at right angles to the ideal light rays that describe the direction of propagation.

Transverse waves commonly occur in elastic solids due to the shear stress generated; the oscillations in this case are the displacement of the solid particles away from their relaxed position, in directions perpendicular to the propagation of the wave. These displacements correspond to a local shear deformation of the material. Hence a transverse wave of this nature is called a shear wave. Since fluids cannot resist shear forces while at rest, propagation of transverse waves inside the bulk of fluids is not possible. In seismology, shear waves are also called secondary waves or S-waves.

Transverse waves are contrasted with longitudinal waves, where the oscillations occur in the direction of the wave. The standard example of a longitudinal wave is a sound wave or "pressure wave" in gases, liquids, or solids, whose oscillations cause compression and expansion of the material through which the wave is propagating. Pressure waves are called "primary waves", or "P-waves" in geophysics.

Water waves involve both longitudinal and transverse motions.

Mathematically, the simplest kind of transverse wave is a plane linearly polarized sinusoidal one. "Plane" here means that the direction of propagation is unchanging and the same over the whole medium; "linearly polarized" means that the direction of displacement too is unchanging and the same over the whole medium; and the magnitude of the displacement is a sinusoidal function only of time and of position along the direction of propagation.

The motion of such a wave can be expressed mathematically as follows. Let ${\displaystyle {\widehat {d}}}$ be the direction of propagation (a vector with unit length), and ${\displaystyle {\vec {o}}}$ any reference point in the medium. Let ${\displaystyle {\widehat {u}}}$ be the direction of the oscillations (another unit-length vector perpendicular to d). The displacement of a particle at any point ${\displaystyle {\vec {p}}}$ of the medium and any time t (seconds) will be $${\displaystyle S({\vec {p}},t)=A\sin \left((2\pi ){\frac {t-{\frac {({\vec {p}}-{\vec {o}})}{v}}\cdot {\widehat {d}}}{T}}+\phi \right){\widehat {u}}}$$ where A is the wave's amplitude or strength, T is its period, v is the speed of propagation, and ${\displaystyle \phi }$ is its phase at t = 0 seconds at ${\displaystyle {\vec {o}}}$. All these parameters are real numbers. The symbol "•" denotes the inner product of two vectors.

By this equation, the wave travels in the direction ${\displaystyle {\widehat {d}}}$ and the oscillations occur back and forth along the direction ${\displaystyle {\widehat {u}}}$. The wave is said to be linearly polarized in the direction ${\displaystyle {\widehat {u}}}$.