Original Kingdom Hearts design documents reveal many differences between early game concepts to those that appear in the final game. These include the appearance of a strange icon depicting a drastically different Destiny Islands, unused Heartless designs, and Sora having clothes similar to Tarzan in Deep Jungle.

Kathryn Beaumont, the original voice of Alice in Alice in Wonderland and Wendy in Peter Pan, reprised her role as Alice and Wendy in Kingdom Hearts at the age of 64.

A red Trinity can be found in Halloween Town that must be collected before Spoiler:Oogie Boogie is defeated, otherwise it will become impossible to collect and the player will be unable to get a 100% completion rating in Jiminy's Journal. The red trinity is located Spoiler:inside Oogie's Manor, which disappears when Oogie is defeated. This was later fixed on the Final Mix release of the game by relocating the Trinity to the ground.

After the release of the first Kingdom Hearts game, Disney was planning on making an animated series based on the game. A full-color, voice-acted animatic pilot for the show was produced and received well, but ultimately the show was never made. Since Kingdom Hearts: Chain of Memories and Kingdom Hearts II were formally announced around the time the pilot was made, Disney opted for Square-Enix to continue the story with more games rather than through an in-house animated series.

In 2022, the animatic pilot was uploaded online by its director Seth Kearsley.

In Kingdom Hearts, the title card for Halloween Town contains a faster, pitched up version of Kefka's laugh from Final Fantasy VI. This detail reappears in Kingdom Hearts II, although it is slowed and pitched down slightly to be easier to make out.

The small figurine inside Ariel's Grotto will play a melody ("Who Am I?") from the Final Fantasy VII soundtrack.

In an interview with the Director of Kingdom Hearts, Tetsuya Nomura, he stated that Cloud's appearance in Kingdom Hearts was inspired by Final Fantasy VII character Vincent Valentine. He also stated that he wanted to include Vincent Valentine in the first game.

The Pride Lands were originally going to be to be a playable level, with Sora turning into a lion to fight. This idea was scrapped as fighting and walking in a four-legged body required additional programming that the team did not have time to create. The concept was later used in the sequel.

In an early trailer for the game, there is a scene showing Riku on top of Big Ben in Neverland. This scene does not appear in the final game, nor does Riku ever go to Big Ben in any way. Despite this, it is considered canon as a cutscene in Kingdom Hearts Re:coded shows that this is where Riku and Kairi went after leaving Captain Hook's ship.

The idea for the game came from an elevator ride with Shinji Hashimoto (Final Fantasy and Square producer) and a Disney producer, who shared the same building in Tokyo. They came up with a crossover as a vague production idea, and the head of Square-Enix, Hironobu Sakaguchi, later agreed to the idea.

An early trailer reveals that Donald and Goofy's costumes were originally different.

According to series creator Tetsuya Nomura, the original data and assets for Kingdom Hearts had been lost, forcing Kingdom Hearts HD 1.5 Remix to be remade from the ground up:

Whenever Sora wins a round in the Olympus Colosseum, he performs a small victory pose. These animations are based off of Cloud's victory poses in Final Fantasy 7, and Squall's from Final Fantasy 8.

The ending to the beta trailer for the game shown at E3 2001 featured an alternate logo. The word "Kingdom" is shown in normal golden text while the word "Hearts" is displayed in a modified version of the final logo's spiky text. The trademark heart is also missing from this version of the logo.

The enemy "Assault Rider" was meant to appear in the game but its style didn't fit any of the worlds so it was removed and used in Kingdom Hearts 2 instead.

Completing the world "Deep Jungle" before Wonderland will cause Snow White to be kidnapped instead of Alice in the cut scene which is shown after completing Deep Jungle.

In Deep Jungle, outside of the tents on a table is a pot and cup which bare a resemblance to Ms. Potts and Chip from Beauty and the Beast. These characters also appeared in the Tarzan film.

There was a planned boss battle with Riku who would be possessed by Ansem in an area titled Crumbling Island at the End of The World which was based off of Destiny Island, but devoured by the darkness.

If you enter the Hercules Cup time trial, get to Hercules, pause the game and choose to restart the time trial, it will take you back to the start of the Hercules fight and the timer will be frozen at 00'06"23.

In the Japanese version of Kingdom Hearts, the Chernabog fight does not have the "Night on Bald Mountain" music. Instead the regular boss music will play. The "Night on Bald Mountain" music was added to the international release to make the scene more true to the scene from Fantasia. The music was added into the Final Mix version of the game.