The Grandmaster is the secondary antagonist of Thor: Ragnarok. He is the ruler of the planet Sakaar who enjoys putting life-forms into gladiatorial fights for his own amusement.
He is portrayed by Jeff Goldblum.
In the film, the Grandmaster dedicated much of his desire to win the people's hearts by using different life-forms in a gladiatorial event (known as the Contest of Champions) held in a large arena. He is aided by his personal bodyguard Topaz, and took in a sole Valkyrie dubbed 142 as a new bodyguard following the massacre of Valkyries committed by Hela (the Asgardian Goddess of Death).
The Grandmaster even got 142 to successfully capture the Hulk (who exiled himself from Earth following the deaths of Ultron and the Ultron Sentinels two years ago), and he made Hulk his new champion due to the latter's super strength that has admired much of the Sakaarian people.
When Thor and Loki end up being banished from Asgard by Hela, they end up crash-landing on Sakaar, where Thor is enslaved and becomes a gladiator while Loki manages to win the Grandmaster's favor and becomes an ally of his. As the Grandmaster and Loki arrange a fight between Thor and the Hulk, the Grandmaster informs Thor that he will grant his freedom if he defeats the Hulk. Despite some reluctance, Thor almost defeats the Hulk, but the Grandmaster secretly sabotages the fight by using the paralyzing chip in his neck to ensure the Hulk's victory, as he has no intention in setting Thor free.
Eventually, Thor manages to escape from confinement and freed the Hulk from the Grandmaster's control by using a recording of Black Widow. Outraged by this action, the Grandmaster orders the Sakaaran Guards to capture both Thor and the Hulk, but after reliving the deaths of her fellow Valkyries (thanks to Loki's magic), 142 decides to help Thor in defeating Hela, even capturing Loki to prove her goodwill.
Working together, Thor, Hulk and 142 staged a revolution against the Grandmaster and his forces by freeing the other gladiators from confinement. After sending Topaz to her death, Thor, Hulk and 142 head over to Asgard through a wormhole in order to save the surviving Asgardians from Hela and her army of Berserkers. Even Loki and several gladiators provided one of the Grandmaster's giant ships to evacuate the Asgardians as Loki summons the fire demon Surtur (per Thor's orders) to destroy all of Asgard, killing Hela and the Berserkers in the process.
Following the film's end credits, the Grandmaster is last seen being confronted by several of the rebels, who have now overthrown him after defeating his forces. He attempts to appease the rebels by taking credit for the revolution and claiming that a revolution cannot be had without having someone to overthrow, declaring the revolution "a tie". However, it is quite clear that the rebels are not convinced by this, much to the Grandmaster's distraught. It can be implied that the Grandmaster is either imprisoned by the rebels or possibly executed.