Gregory Donovan Ostertag (born March 6, 1973, in Dallas, Texas) is a retired American basketball player in the NBA, who spent most of his career with the Utah Jazz. He measures 7'2" (2.18 m) and played center. Throughout his college and professional career he wore the unusual uniform number 00 (but did wear no. 39 for two seasons).

After graduating from Duncanville High School (where he had already grown to seven feet, and led the school to its first boys basketball title in 1990), Ostertag played college basketball for the University of Kansas from 1991 through 1995, playing in two Final Fours, and swatting 258 career shot blocks during that time, a record for both Kansas and the Big Eight Conference. He was drafted by the Utah Jazz in the first round in 1995. He spent the next 9 seasons playing for the Jazz, playing alongside superstars John Stockton and Karl Malone, before signing as a free agent with the Sacramento Kings in 2004. His season with the Kings was unproductive as he had little playing time. Later in his career, on August 2, 2005, Ostertag was involved in a 5-team, 13-player trade (the largest in NBA history) that sent him back to the Jazz.

After the 2005-06 season, Greg Ostertag announced his retirement from the Utah Jazz. His last game was on April 19th, the season finale versus the Golden State Warriors. Utah won the game 105-103 and Ostertag received the opportunity to play for 3:36, registering one rebound and one block. The win against the Golden State Warriors ensured that Ostertag played his entire career on teams that had winning seasons.

Off the court

In 2002, he donated a kidney to save the life of his sister Amy Hall, who was dying of kidney disease; upon his return he became the first player in NBA history to play after donating an organ (Sean Elliott had returned to play earlier; however, he was the first organ transplant recipient to return to NBA action). He has since been an advocate of organ donation and the Kidney Foundation. He is currently in fourth place in number of games played for the Jazz, behind Stockton, Malone, and Mark Eaton.