Diagonal argument can refer to:

• Diagonal argument (proof technique), proof techniques used in mathematics.

A diagonal argument, in mathematics, is a technique employed in the proofs of the following theorems:

• Cantor's diagonal argument (the earliest)
• Cantor's theorem
• Russell's paradox
• Diagonal lemma
  • Gödel's first incompleteness theorem
  • Tarski's undefinability theorem
• Halting problem
• Kleene's recursion theorem

• Diagonalization (disambiguation)