Friedrich Waismann (March 21, 1896 - November 4, 1959) was an Austrian mathematician, physicist, and philosopher. He is best known for being a member of the Vienna Circle and one of the key theorists in logical positivism

Born in Vienna, Waismann was educated in mathematics and physics at the University of Vienna. In 1922, he began to study philosophy under the tutelage of Moritz Schlick, the founder of the Vienna Circle. He emigrated to the United Kingdom in 1938. He was a reader in philosophy of science at the University of Cambridge from 1937 to 1939, and lecturer in philosophy of mathematics at the University of Oxford from 1939 until his death.

Intermittently, from 1927 until 1936, Waismann had extensive conversations with Ludwig Wittgenstein about topics in philosophy of mathematics and philosophy of language. These conversations, recorded by Waismann, were published in Ludwig Wittgenstein and the Vienna Circle (1979, ed. B.F. McGuinness). Other members of the Circle (including Schlick, Rudolf Carnap, and Herbert Feigl) also spoke with Wittgenstein, but not to Waismann's extent. At one point in 1934, Wittgenstein and Waismann considered collaborating on a book, but these plans fell through after their philosophical differences became apparent.

In Introduction to Mathematical Thinking: The Formation of Concepts in Modern Mathematics (1936), Waismann argued that mathematical truths are true by convention rather than being necessarily (or verifiably) true. His collected lectures, The Principles of Linguistic Philosophy (1965), and How I See Philosophy (1968, ed. R. Harré), a collection of papers, were published posthumously.