also: うんめいろんしゃ
noun
A person who believes that all events are predetermined and inevitable. Often used in philosophical or religious contexts.
彼は運命論者だから、努力しても無駄だと思っている。
He is a fatalist, so he thinks effort is futile.
運命論者の考え方では、未来は変えられない。
In the thinking of a fatalist, the future cannot be changed.
Compound of 運命 (fate) + 論 (theory, argument) + 者 (person). The reading じゃ is the standard on'yomi for 者 in such compounds.