In 2013, Square Enix published a novella titled "Final Fantasy X-2.5 ~Eien no Daishō~" ("Price of Eternity"), to commemorate the release of the Final Fantasy X/X-2 HD remasters that year. It was written by both games' scenario writer Kazushige Nojima and never received an official English release. The only existing fan translation as of 2024 was translated from Japanese to French to English. The book was heavily criticized worldwide for its vague writing style, introducing violent and sexual elements uncommon to the Final Fantasy series, and weakening the impact of both games' endings. For context, X's plot builds up to Spoiler:a negative ending where Tidus dies and Yuna has to accept his death and move on, while X-2 turns it back around to Spoiler:a positive ending where Tidus is brought back to life. What X-2.5 does to weaken this is Spoiler:immediately kill Tidus and just as quickly resurrect him for the express purpose of setting up future stories, removing the emotional weight and finality from him being truly dead or alive.

X-2.5's plot takes place between the first ending of X-2 and its secret ending. It has two sections written from Tidus and Yuna's own perspectives and a third section with both perspectives. After reuniting, Yuna briefly leaves to address recent developments on Besaid Island. Tidus later goes sailing to look for her, falls asleep, and wakes up finding her in the boat. On their way back to Besaid, Spoiler:they get lost in a storm and are marooned on an island resembling Besaid, having transported 1000 years back in time. While there, Tidus inexplicably gets hit in the head with an object resembling a Blitzball that is actually a bomb. When he goes to pick it up, it explodes, decapitating Tidus and sending his head flying near Yuna with a surprised expression on his face. She faints and sees a vision of a god-like figure who helps her resurrect a ghost-like version of Tidus through Beckoning, where someone thinks of a person they knew who died, summoning an illusion of them from the Farplane made of Pyreflies, but if the illusion learns they are not actually alive, they will fade away.

The rest of the story involves Yuna Spoiler:going back and forth from the past to present trying to keep Tidus' spirit from vanishing by interacting with new characters. These include Briar and Kush, who also turn out to be Beckoned spirits from the past and vanish at the end of the book, and Ifarnal, a summoner who tells Yuna that Tidus must kill Kush if they want to return to the present together and later gets killed by another bomb. Through the subplot with the new characters, it's established that one way to create "cores" used to summon Aeons (these Aeon Cores are said to be very similar to Faytes but have differences) is implied to be through consensual sex in two scenes. One where Yuna is transferred the knowledge on how to create them and blushes in embarrassment, and one where Briar discovers Ifarnal and a half-naked Kush in bed together. At the end of the book, there is a small time-skip to the secret ending of X-2, shown in a different context now knowing the events of X-2.5, and ends on a "To Be Continued" cliffhanger.

Final Fantasy X-2: Last Mission takes place after X-2.5. Since it was released years before, it does not acknowledge the events of X-2.5, but the HD remaster does include a continuation that is relevant to both. It is a 30-minute audio drama that was dubbed internationally called "Final Fantasy X -Will-" and was also written by Nojima. The story follows two summoners, Chuami and her assistant Kurgum, who gradually meet the cast of X/X-2. During this, Yuna discovers Spoiler:that Sin is among a horde of souls being beckoned back from the Farplane. In what seems to be an effort to prevent the still undead Tidus from sacrificing himself to kill Sin again, she breaks up with him by lying about being in love with another man, and the summoners go to fight Sin, appearing to set up the plot for a potential Final Fantasy X-III.

In a 2001 interview with the game's character designer Tetsuya Nomura published in the V-Jump magazine, he stated that he used the same approach to making character designs from Final Fantasy VIII for this game. The first thing he did was draw just the character's face, and then he discusses with the team on how "realistic" the design should be. After that he'd asked about the backstories for each character and use that to figure out the details of their designs; for example asking if they were an athletic, sportsy type, or are they are a summoner type of character. Nomura also stated that a big consideration for the game was that the world has an "Asian" vibe to it, so he tried to be conscious of that style while at the same time making sure the main characters didn't look too familiar between each other. The team asked Nomura that he only made the main protagonist Tidus stand apart somewhat from the overall world, so Tidus' design is very distinct from the Asian aesthetic.

According to a 2001 developer interview published in V-Jump magazine, the game's character designer Tetsuya Nomura stated the main character's name, Tiida (Tidus in the international release), was the Okinawan word for "sun". After Final Fantasy VIII, the team wanted to give their next main character a brighter and more cheerful personality. Kazushige Nojima gave Tiida his name, and Nomura thought he had a "bright" motif for the character in mind considering his name's meaning.

At the 2017 Emerald City Comic Con during a panel with American voice actor John DiMaggio, he explained that he was told to make Wakka's voice sound like an islander, with an island accent. At the time, he broke up with his girlfriend from Hawaii, so he came up with some Hawaiian impersonations which landed him the role of Wakka.

According to a 2001 developer interview published in V-Jump magazine, the game's composer Nobuo Uematsu stated his team started including theme songs in the Final Fantasy games since Final Fantasy VIII, and that each time, Uematsu & Yoshinori Kitase would hold a meeting to decide who would sing the theme song. The developers brought in various music CDs of singers they liked to listen to and would share their opinions. At some point during the meeting for Final Fantasy X's theme song, they could not decide on who to pick. Around the same time, a member from their staff happened to buy one of Rikki’s independent releases at a CD shop, and showed it to Uematsu. He loved it after listening to it, and said in a excited mood "This is great!", leading the team to contact Rikki right away to perform the song, called "Suteki Da Ne", for the game. Uematsu considered it a memorable song for FFX, and that from the beginning the music would be a central part of the soundtrack.

According to a 2001 V-Jump magazine interview with the game's designer Fumi Nakashima, she stated that her inspiration for the Al Bhed’s clothing came from bondage fashion. She recalled having various bondage books and magazines on her desks at work and her co-workers giving her "weird looks" in response to seeing them.

In a 2001 V-Jump magazine interview with the game's art director Yusuke Naora, he was asked where the idea for an "asian" themed world came from? He responded:

In a 2001 V-Jump magazine interview with the game's character designer Tetsuya Nomura, he said that when designing Tidus' emblem, he thought of various things related to the ocean, like fishing hooks, fishbones, whale fins, and based his designs about them. He mentioned that Tidus' necklace is the emblem for his blitzball team, the Zanarkand Abes. And also stated that he came up with the idea before the Al Bhed language was written, but the shape design of the necklace is actually the combined letters of "T" + "J" from the start of Tidus & Jecht's names.

Auron's voice actor, Matt McKenzie, is also the voice of Major Elliot from Final Fantasy: The Spirits Within.

Final Fantasy X, like other games in the series, features many references to real world religions and their history. This game in particular alludes to the conflicts between Christianity and Gnosticism; it also depicts the corruption possible within organized religion, and how it can lead to an inversion of its own doctrine.

During the fight with Spoiler:Seymour Omnis inside of Sin, summoning Anima will provoke a response from Seymour, "You would oppose me as well? So be it." He says this because Anima was formerly his final aeon, as well as his mother.

When you talk to the woman taking care of the Chocobos at the bottom of the ship during the sea voyage to Kilika, she will sing a part of the Chocobo Theme.

Cloud's Buster Sword was originally intended for use by Tidus but was removed before the game was released. It is still in the game's code, though, and using hacking tools, it can be obtained and used in the final version.

Lulu's dress in Final Fantasy X was designed by Tetsuya Nomura specifically to see if the graphic designers could keep the exact number of belts in the right places every time. This is why many cutscenes with her show Lulu from the waist up or from behind.

In Tidus's early development, he originally had darker colored hair. It was changed, most likely because his name means "Sun" in Okinawan, and the developers wanted the color of his hair to reflect that.

Character designer Tetsuya Nomura has said that the South Pacific, Thailand and Japan are major influences on the culture and geographical design of Spira, particularly in the case of the southern islands of Besaid and Kilika.

Lulu's starting weapon, a Moogle Doll, is the only appearance of a Moogle in the game, a massive difference from all previous titles in the series. Her other weapons are references to previous characters in the series, including her final weapon of the Onion Knights from Final Fantasy III.

Final Fantasy X was the first core Final Fantasy title that wasn't entirely composed by Nobuo Uematsu. The score was also created by Masashi Hamauzu and Junya Nakano.

At the end of the game Yuna will proclaim to Tidus that she loves him. However in the Japanese version Yuna will simply thank him. This change was to make up for cultural differences.

Tidus is never actually directly referred to by his name in audible dialogue. People will refer to him with impersonal pronouns. This is due to the player being able to name Tidus at the start of the game, and this being Square Enix's first voiced Final Fantasy game, they weren't sure how to handle a variable name. Tidus' name pronunciation has been debated, but was confirmed to be pronounced "Tee-Dus" by James Arnold Taylor, the voice actor for Tidus.