Paul Richard Halmos (born March 3, 1916) is a Hungarian-born American mathematician who has done research in the fields of probability theory, statistics, operator theory, ergodic theory, and functional analysis (in particular Hilbert spaces).

He is noted for a number of expository books, viewed by many to be well written, including Naive Set Theory, Introduction to Hilbert Space and the Theory of Spectral Multiplicity , Lectures on Boolean Algebras , and Finite-Dimensional Vector Spaces . His autobiography, published in 1987, is titled I Want to Be a Mathematician .

The use of "iff" to abbreviate "if and only if" is sometimes mistakenly credited to Halmos; however, he has said that he borrowed this notation. The use of the "tombstone" notation to signify the end of a proof is also credited to him; the tombstone symbol ∎ is sometimes called a halmos.

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