**Thomas J. Jech** (Czech: *Tomáš Jech*, pronounced [ˈtomaːʃ ˈjɛx]; born 29 January 1944 in Prague) is a mathematician specializing in set theory who was at Penn State for more than 25 years.

He was educated at Charles University (his advisor was Petr Vopěnka) and from 2000 is at the Institute of Mathematics of the Academy of Sciences of the Czech Republic.

Jech's research also includes mathematical logic, algebra, analysis, topology, and measure theory.

Jech gave the first published proof of the consistency of the existence of a Suslin line.
With Karel Prikry, he introduced the notion of precipitous ideal. He gave several models where the axiom of choice failed, for example one with ω_{1} measurable. The concept of a Jech–Kunen tree is named after him and Kenneth Kunen.