(covers information from several alternate timelines)
Khitomer was an inhabited planet in the Khitomer system of the Beta Quadrant. Khitomer played a vital role in the development of the Klingon-Federation alliance, being the site of the Khitomer Conference of 2293 as well as the victim of a Romulan sneak attack on the planet's Klingon colony in 2346, which, in turn, emphasized the importance of the Klingon-Federation friendship. (Star Trek VI: The Undiscovered Country; TNG: "The Neutral Zone", "Sins of the Father")
In the alternate 2259, the planet was discovered to have a Klingon colony. This event was reported in a news ticker, which was in a collection of graphics and video media that were seen on a powerwall in the offices of Christopher Pike and Alexander Marcus at Starfleet Headquarters. (Star Trek Into Darkness)
In 2293, Khitomer was considered to be a neutral site by both the Federation and the Klingon Empire, and was located near the Romulan border. In that year, the Khitomer Conference was held at Camp Khitomer on the planet. The conference started the first full peace negotiations between the United Federation of Planets and the Klingon Empire and gave birth to the Khitomer Accords. Originally, Earth had been selected as a venue, but after Klingon chancellor Gorkon was assassinated on his way to Earth by members of the Khitomer conspiracy, new chancellor Azetbur relocated the conference to Camp Khitomer. (Star Trek VI: The Undiscovered Country)
By 2346, a Klingon outpost-colony existed on Khitomer. On stardate 23859.7, the colony fell victim to a Romulan attack. In the attack – which later became known as the Khitomer Massacre – 4,000 colonists were killed, among them Mogh and Ja'rod (TNG: "Heart of Glory", "Sins of the Father"). Only two survivors were believed to have been rescued by the USS Intrepid; however, a group of colonists captured by the Romulans were allowed to live and were taken to a prison camp in the Carraya sector, remaining undiscovered until 2369. (TNG: "Birthright, Part I", "Birthright, Part II")
- See: Khitomer inhabitants
This planet's quadrant of origin is inferred based on the position of its star system as seen in the star charts appearing in the Star Trek: Discovery episode DIS: "Magic to Make the Sanest Man Go Mad" and in the Star Trek: Picard episode "Maps and Legends".
The script for "Sins of the Father" had Kurn state to Worf that the Khitomer Outpost was "new... unfinished" at the time that Mogh visited. 
During the events of the Star Trek: Destiny novel Gods of Night, Khitomer was saved from annihilation by the Borg in 2381 by a kamikaze attack from the USS Ranger (β). But in the novel Lost Souls the planet was again attacked and fell to the Borg Invasion of 2381 (β). None of the population survived this attack, including presumably Governor Talgar (β).
In Star Trek Online, the Klingon Empire reclaimed the Khitomer sector in 2382 during the political crisis in the Romulan Star Empire that followed Praetor Shinzon's death. In the mission "Turning Point" the planet was the site of a conference between the Federation, the Klingon Empire, and the Romulan Star Empire, regarding the future of the Romulan Republic (β). The conference center later becomes the headquarters of the Khitomer Alliance (β) (formed by the Federation, the Klingon Empire, the Romulan Republic, and the Dominion, among others) until it is destroyed by a mycelium weapon in "Khitomer Discord".
According to the RPG sourcebook Worlds, Khitomer was the third planet in the Beta Guarada system.
In the Star Trek: Ongoing story arc The Khitomer Conflict set in the alternate reality, construction of the Khitomer colony was underway in the year 2261, but on stardate 2261.147, a Romulan fleet led by Commander L'Nar (β) attacked and destroyed the settlement while it was still under construction. The USS Enterprise responded to the distress call about the attack and a landing party led by James T. Kirk visited the colony site but found no survivors. They end up captured by Commander Kor, who believed the Federation was responsible for the attack. While the Enterprise is forced to flee, Kirk and his officers were brought to Qo'noS as prisoners. The brazen attack leads to the Khitomer Conflict between the three major powers. It is eventually revealed that Section 31 traded advanced weaponry to the Romulans in an attempt to destroy the Klingon Empire, but Section 31 double-crossed the Romulans by installing a self-destruct program onto the modified warbirds, which they destroyed. Section 31 had accomplished their goals; they had the Klingon and Romulan fleet cut to pieces, the red matter in their possession, and assurances that the Klingons would retaliate against the Romulans, all to the Federation's advantage.