Thomas Johann Seebeck (9 April 1770 – 10 December 1831) was a physicist who in 1821 discovered the thermoelectric effect.

Seebeck was born in Reval (today Tallinn, Estonia) to a wealthy Baltic German merchant family. He received a medical degree in 1802 from the University of Göttingen, but preferred to study physics. In 1821 he discovered the thermoelectric effect, where a junction of dissimilar metals produces an electric current when exposed to a temperature gradient. This is now called the Peltier–Seebeck effect and is the basis of thermocouples and thermopiles.

In 1821 Thomas Johann Seebeck found that a circuit made from two dissimilar metals, with junctions at different temperatures would deflect a compass magnet. Seebeck initially believed this was due to magnetism induced by the temperature difference. However, it was quickly realized that it was an electrical current that is induced, which by Ampere's law deflects the magnet. More specifically, the temperature difference, produces an electric potential (voltage) which can drive an electric current in a closed circuit. Today, this effect is known as the Peltier–Seebeck effect.

The voltage produced is proportional to the temperature difference between the two junctions. The proportionality constant (a) is known as the Seebeck coefficient, and often referred to as the thermoelectric power or thermopower. The Seebeck voltage does not depend on the distribution of temperature along the metals between the junctions. This effect is the physical basis for a thermocouple, which is used often for temperature measurement.

The voltage difference, V, produced across the terminals of an open circuit made from a pair of dissimilar metals, A and B, whose two junctions are held at different temperatures, is directly proportional to the difference between the hot and cold junction temperatures, T_h− T_c.

In 1810, at Jena, Seebeck described the action of the spectrum of light on the chloride of silver. He observed that the exposed chemical would sometimes take on a pale version of the color of light that exposed it, and also reported the action of light for a considerable distance beyond the violet end of the spectrum. Seebeck also worked on the theory of color with Johann Wolfgang von Goethe.

In 1808, Seebeck was first to produce and describe the amalgam of potassium. In 1810, he observed the magnetic properties of nickel and cobalt. In 1818, Seebeck discovered the optical activity of the solutions of sugar.