Aloysius Martin Thesz (April 24, 1916 – April 28, 2002), known by the ring name Lou Thesz, was an American professional wrestler and wrestling coach. Considered to be one of the last true shooters (legitimate wrestlers) in professional wrestling and described as the "quintessential athlete" and a "polished warrior who could break a man in two if pushed the wrong way", Thesz is widely regarded as one of the greatest wrestlers and wrestling world champions in history, and possibly the last globally accepted world champion.

Thesz won various version of the World Heavyweight Championship seven times, including three reigns as NWA World Heavyweight Champion, which he held for a combined total of ten years, three months and nine days (3,749 days) – longer than anyone else in history. In Japan, Thesz was known as the "God of Wrestling'" (like his Belgian counterpart, Karl Gotch) and was called "Tetsujin", which means "Ironman", in respect for his speed, conditioning and expertise in catch wrestling. Alongside Karl Gotch and Billy Robinson, Thesz later helped train young Japanese wrestlers and mixed martial artists in catch wrestling. In his final years, he was head coach at the CACC Snake Pit Japan [ja].

A successful amateur wrestler in his youth and an ardent supporter of the sport in his later years, he helped establish, in addition to being a member of its inaugural class, the George Tragos/Lou Thesz Professional Wrestling Hall of Fame, which honors successful professional wrestlers with a strong amateur wrestling background, and is a charter member of several other halls of fame, including: WCW, Wrestling Observer Newsletter, Professional Wrestling and WWE's Legacy Wing.

Aloysius Martin Thesz was born in Banat, Michigan on April 24, 1916. His father, Martin, was a working-class shoemaker of Hungarian and German descent; his mother, Katherine (née Schultz), also of German descent, hailed from the Austro-Hungarian Empire. His family moved to St. Louis in Missouri when he was a young boy. Thesz and his three sisters grew up speaking German at home and he did not start learning English until he entered kindergarten at age five. Hungarian was also spoken in the Thesz household but the children did not learn it. In addition to public school, he also had to attend German school every Saturday until he was eight. He was fluent in German and English.

Thesz's father was a national Greco-Roman wrestling middleweight champion in his native Hungary and introduced Lou to the sport as a young boy. At eight years old, Lou began a tough and thorough education in Greco-Roman wrestling under his father, which provided the fundamentals for his later success. He trained in Greco-Roman wrestling under the guidance of his father for several years until transitioning to folkstyle wrestling in high school, where he was a successful competitor on his school team. He also trained in boxing as a teenager. Thesz dropped out of high school by age 14 to work at his father's shoe repair business and began training in freestyle wrestling at Cleveland High School due to his father knowing the wrestling coaches. He quickly became an accomplished freestyle wrestler, competing in city-wide intramurals and regional tournaments in the 160 lb division. At aged 16, Thesz then gained further training in freestyle wrestling under John Zastro. Thesz credited Zastro for elevating his wrestling and became his regular sparring partner. Thesz won several amateur titles and became one of the most dominant freestyle wrestlers of his weight class in the county, and caught the eye of Tom Packs, a professional wrestling promoter in St. Louis. Packs met with Thesz and asked if he wanted to wrestle professionally and Thesz accepted. Thesz later said he would have continued his amateur career had he not been asked. Packs sent Lou to George Tragos for further coaching.

George Tragos, a feared Greek Olympic freestyle wrestler, catch wrestler and wrestling coach at the University of Missouri, took a liking to Thesz and respected his willingness to work hard and follow instruction. He trained under the watchful eye of Tragos for nearly four years at the National Gym in St. Louis. Tragos, who had a well-deserved reputation as a dangerous catch wrestler, decried the emerging performance-related aspects of the sport and instead coached young men to be true, authentic professional wrestlers. Tragos specifically taught Thesz submission wrestling and how to wrestle from the bottom. Thesz remembered Tragos saying, "any fool can start on top. If you start at the bottom, you learn to wrestle." Ray Steele also served as a coach and mentor to Thesz during this time.

Thesz also studied under German-born catch wrestler Ad Santel, who was known for his feud with the Kodokan judo school. Thesz studied under Santel for up to five days every week during a 6-month stay in California and remembered it being the "most intensive training period of my life". Throughout his career, Thesz continued to train under Santel whenever he was in the California area. The training he received under Santel would help establish Thesz as one of the most dangerous grapplers in the world.

Thesz later met legendary former champion Ed "Strangler" Lewis in St. Louis and was encouraged to challenge Lewis to a friendly contest. Although Thesz lost the 15-minute contest, Lewis was impressed by Thesz's skills and later became his manager and trainer. Lewis later described Thesz as "lithe as a panther and exceptionally fast. He moves with the speed of a lightweight." As his trainer, Lewis taught Thesz extremely painful and potentially crippling submission holds that would help him when facing opponents that refused to lose.