In logic and mathematics, diagonalization may refer to:

• Matrix diagonalization, a construction of a diagonal matrix (with nonzero entries only on the main diagonal) that is similar to a given matrix
• Diagonal argument (disambiguation), various closely related proof techniques, including:
  • Cantor's diagonal argument, used to prove that the set of real numbers is not countable
  • Diagonal lemma, used to create self-referential sentences in formal logic
• Table diagonalization, a form of data reduction used to make interpretation of tables and charts easier.