In Euclidean geometry, a **harmonic quadrilateral**, or **harmonic quadrangle**,^{[1]} is a quadrilateral that can be inscribed in a circle (cyclic quadrangle) in which the products of the lengths of opposite sides are equal. It has several important properties.

Let ABCD be a harmonic quadrilateral and M the midpoint of diagonal AC. Then:

- Tangents to the circumscribed circle at points A and C and the straight line BD either intersect at one point or are mutually parallel.

- Angles ∠BMC and ∠DMC are equal.

- The bisectors of the angles at B and D intersect on the diagonal AC.

- A diagonal BD of the quadrilateral is a symmedian of the angles at B and D in the triangles ∆ABC and ∆ADC.
- The point of intersection of the diagonals is located towards the sides of the quadrilateral to proportional distances to the length of these sides.