Etienne-Louis Malus (July 23, 1775 - February 24, 1812) was a French officer, engineer, physicist, and mathematician.

Malus was born in Paris, France. He participated in Napoleon's expedition into Egypt, 1798 to 1801. Malus became a member of the French academy of sciences in 1810.

His mathematical work was almost entirely concerned with the study of light. This involved him in studying geometrical systems called ray systems, closely connected to Plücker's line complexes. He conducted experiments to verify Huygens' theories of light and rewrote the theory in analytical form. His discovery of the polarisation of light by reflection was published in 1809 and his theory of double refraction of light in crystals in 1810.

He is probably best known now for "Malus' Law", the law giving the intensity produced when a polarizer is placed in front of an incident beam:

I = I₀ cos² θ

where θ is the angle between the polarisation direction of the incident beam and the axis of the polariser.