John Albert Kramer (August 1, 1921 – September 12, 2009) was an American tennis player of the 1940s and 1950s, and a pioneer promoter who helped drive the sport towards professionalism at the elite level. Kramer also ushered in the serve-and-volley era in tennis, a playing style with which he won three Grand Slam tournaments (the U.S. Championships in 1946 and 1947, Wimbledon in 1947). He also led the U.S. Davis Cup tennis team to victory in the 1946 and 1947 Davis Cup finals.

Kramer won the U.S. Pro Championship at Forest Hills in 1948 and the Wembley Pro Championships in 1949. He won world professional championship 2-man tours in 1948 (against Bobby Riggs), 1949/50 (against Pancho Gonzales), 1950/51 (against Pancho Segura), and 1953 (against Frank Sedgman).

Kramer was ranked world no. 1 amateur player for 1946 by Pierre Gillou, Harry Hopman and Ned Potter. He was ranked world no. 1 amateur player for 1947 by John Olliff, Pierre Gillou and Ned Potter. In 1948 he was ranked the U.S. no. 1 professional in the USPLTA contemporary rankings for U.S. pro tennis play. Some recent tennis writers have considered Kramer to be the world no. 1 player from 1946 to 1953, spanning his last amateur years and early pro years.

He was the leading promoter of professional tennis tours in the 1950s and 1960s, signing many of the most accomplished tennis players of the era to professional contracts. Kramer played an important role in the establishment of modern men's Open-era tennis.

Kramer began his tennis career by taking lessons from renowned teaching professional, Dick Skeen. Within a year, he was playing junior tournaments. He played on the Montebello High School tennis team with George Richards. Because of his obvious ability and his family's lack of money, he came under the guidance of Perry T. Jones. at the Los Angeles Tennis Club (LATC). Jones was the President of the Southern California Tennis Association (SCTA). Kramer traveled many hours each day from his home in Montebello, California, to play tennis at the LATC and the Beverly Hills Tennis Club. He was able to play against such great players as Ellsworth Vines, Bobby Riggs, and Bill Tilden. Kramer was the National Boys' Champion in 1936, and the winner of the 1938 National Juniors Interscholastics. He competed occasionally in men's tournaments on grass courts in the East. He won matches against nationally ranked men such as Elwood Cooke. Kramer also played with high school teammate George Richards, who later was nationally ranked.

Kramer competed at the U.S. National Championships seven times from 1938 through 1947. He lost his first match in 1938 in straight sets, winning only two games. At the 1939 U.S. Championships he was beaten in the second round by 11th-seeded and fellow Californian Joe Hunt. In 1940 Kramer defeated fourth-seeded Frank Parker in a five-set quarterfinal but lost to second-seeded and eventual champion Don McNeill in the semifinal. At the 1941 Championships, he was seeded for the first time, at no. 11, and reached the quarterfinal where third-seeded Frank Kovacs proved too strong. In 1942, Kramer won the men's singles in the Ojai Tennis Tournament. Kramer received a leave from his duties in the United States Coast Guard to compete at the 1943 Championships. Seeded second, he reached the final, despite being weakened by food poisoning, but lost it to Joe Hunt in four sets. During World War II, he continued to win prizes in the United States as the war had effectively put an end to international tennis, but did not compete in the U.S. Championships in 1944 and 1945.

The first Grand Slam tournament Kramer entered after the war was the 1946 Wimbledon Championships where he was seeded second but was upset in by Jaroslav Drobný in a five-set fourth round match. At the 1946 U.S. Championships, he was seeded third but managed to win his first Major singles title after a straight-set victory in the final against Tom Brown, losing just a single set in the tournament. He was ranked world no. 1 amateur in 1946 by Pierre Gillou, Harry Hopman and Ned Potter.

At the 1947 Wimbledon Championships, Kramer was seeded first and justified it by winning the title after another straight-sets win against Brown in a final that lasted only 48 minutes. At 1947 U.S. Championships, number one seed Kramer faced Frank Parker in the final. Parker won the first two sets as Kramer struggled to find form. Then, Kramer "changed suddenly from a stumbling novice to a raging perfectionist" and went on to win in five sets to retain his title. Kramer was ranked world no. 1 amateur in 1947 by John Olliff, Pierre Gillou and Ned Potter.