Masayoshi Nagata (Japanese: 永田 雅宜 Nagata Masayoshi; February 9, 1927 – August 27, 2008) was a Japanese mathematician, known for his work in the field of commutative algebra.

The Chevalley–Iwahori–Nagata theorem describes the quotient of a variety by a group.

In 1959, he introduced a counterexample to the general case of Hilbert's fourteenth problem on invariant theory.

Nagata's conjecture on curves concerns the minimum degree of a plane curve specified to have given multiplicities at given points; see also Seshadri constant.

Recent work has solved this latter problem in the affirmative.