Peter Walden Milonni (born 5 May 1947) is an American theoretical physicist who deals with quantum optics, laser physics, quantum electrodynamics and the Casimir effect.

Milonni earned his PhD in 1974 at the University of Rochester. He then worked at the Air Force Weapons Laboratory from 1974 to 1977, then working at PerkinElmer from 1977 to 1980. In 1980 he became professor of physics at the University of Arkansas; From 1986 to 1994 he was at the Los Alamos National Laboratory, where he became a fellow of the laboratory from 1994 onwards. He then had a research professorship at the University of Rochester.

He is in on editorial boards of Progress in Optics, Contemporary Physics, Advances in Optics & Photonics and Physical Review Letters. Milonni is also author of several textbooks and monographs including The Quantum Vacuum: An Introduction to Quantum Electrodynamics.

In 2008 he received the Max Born Award "For exceptional contributions to the fields of theoretical optics, laser physics and quantum mechanics, and for dissemination of scientific knowledge through authorship of a series of outstanding books".