Martok, son of Urthog is a recurring character in Star Trek: Deep Space Nine, played by actor J. G. Hertzler. Martok is a high-ranking Klingon leader at the Federation-Bajoran space station in the late 2300s. Martok figures prominently in many of the show's long running story arcs, and also is an important relationship for Worf.

J. G. Hertzler made his first appearance in the franchise in the Star Trek: Deep Space Nine pilot episode, "Emissary", as a Vulcan Captain who is killed when his vessel is destroyed. Following this, he made his first appearance as a Klingon in the video game Star Trek: Klingon, directed by Jonathan Frakes and also starring Robert O'Reilly as Chancellor Gowron. When the producers of Deep Space Nine were casting Martok, Hertzler auditioned after initially not considering the role but found himself getting angry during the interview due to a comment from the director which he felt was insulting. However, this response led to him gaining the role, and it was included as a character trait for Martok.

It took around three hours each day for the make-up to be applied to Hertzler by David Quashnick, who was the "specialist" who did both Hertzler and O'Reilly's Klingon make-up. Hertzler did not mind the process, but stated that having the prostheses block the sight in his left side could sometimes make filming difficult as he could not see when turning that way. He felt it was important to the character to remind him of the time spent on the Dominion prison world, and turned down the opportunity to have Martok's sight restored when it was suggested by executive producer Ira Steven Behr. In particular, Hertzler felt it gave the same air to Martok as Christopher Plummer's character in Star Trek VI: The Undiscovered Country.

In 2020, Hertzler talked about developing the character: "As a stage actor you usually get some weeks to rehearse, if you get lucky, and then you go on. And with Martok, I was able to work on him for four years. Now I am still doing it 25 years later!"

In an interview with Heavy in 2021, Hertzler discussed how it was working with the cast of Deep Space Nine. He found it was intimidating at first, because the regular cast had worked with each other for years, and the Martok character had to "boss people around." However, he found the other actors welcomed him and helped him feel like he belonged.

Little backstory is given regarding the early life of Martok, except for a brief history sketched by himself in the Deep Space Nine episode "Once More Unto the Breach". It is known that he was born into a Klingon house (The House of Martok) that was not part of the aristocracy and was raised in the Ket'ha lowlands on the Klingon homeworld of Qo'noS. This area is considered a wasteland by the Klingons.

Martok's family had been loyal soldiers of the Klingon Empire for 15 generations, but never officers. It was Urthog's long-held wish that Martok become an officer, so he enlisted the aid of officers whose respect he had earned, and convinced one to sponsor his son's application to the Klingon academy. They submitted the application to the Oversight Committee, believing that approval would be a mere formality, but was rejected by one member, the legendary Klingon warrior Kor, who held elitist views regarding the honor and prestige of the "great houses". With such a rejection on his record, Martok could no longer even serve as a common soldier. He chose to serve, regardless, and spent five years on General ShiVang's flagship as a civilian laborer. When the ship was surprised by a Romulan attack, Martok took up arms and helped repel the boarding party. His skill and bravery earned him a battlefield commission, but the honour was soured by the fact that Urthog had died some years earlier and couldn't share in his son's glory. Martok later earned the rank of Lieutenant after the Battle of Tcha'voth, becoming tactical officer aboard the cruiser Gothspar under Captain Kultan (ST:DS9 Books: The Left Hand of Destiny), and by his introduction to the series, he had attained the rank of General and still held a grudge against Kor.

Around 2371, Martok was abducted by agents of the Dominion and replaced with a shapeshifter. The exact date of this is unclear. The changeling was successful at beginning the war with the Cardassians (S4, E1) but inadvertently revealed himself to Odo (who had been misled into believing that Chancellor Gowron was the changeling) at a Klingon award ceremony, and was quickly killed by the Klingons attending the ceremony (S5, E1).