noun
A person who competes in a race, especially in sports contexts. Covers track, road, and other racing events.
競走者たちはスタートラインに並んだ。
The runners lined up at the starting line.
選手 is a broader term for an athlete or player in any sport, while 競走者 is limited to racing events.
Compound of 競走 (race, running competition) and 者 (person). The exact historical derivation is uncertain, but the word is transparently formed from these elements.