Thoroughbred racing lost one of its most passionate advocates Tuesday when John Piesen, longtime Post handicapper and writer, died after a brief illness. He was 80.

He is survived by his wife of 38 years, Betsy, five children and four grandchildren.

Piesen began his newspaper career with the Newark News, but moved to The Post in the mid-1970s. He became night sports editor before being assigned to the racetrack.

He quickly established a formidable reputation for picking winners. Piesen’s proud boast was that he selected a winner — often three or four — every single day for a record 207 days. For years, he maintained a friendly but fierce rivalry with another renowned handicapper, the late Russ Harris.

When now-retired Post sports editor Greg Gallo introduced a feature called Thru the Binocs, Piesen became its first writer and it quickly became one of the most-read pages in turf journalism for its wit and sharp observations on horses, trainers and jockeys.

A memorial Mass will be celebrated at Our Lady of Peace in Fords, N.J., at 1 p.m. Saturday.