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

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.

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

According to series producer Tetsuya Nomura, one reason that Mickey Mouse's role was limited in the original Kingdom Hearts was because around the same time as its release, Capcom owned the license to use Mickey Mouse for the game Disney's Magical Mirror Starring Mickey Mouse. The developers persisted and were eventually given the go-ahead to add him into the game "as long as [they only had him in] one scene, from far away, as a silhouette, with him waving his hand or something". As such, Mickey only appears briefly and seen from behind near the end of the game, helping Sora and Riku seal Kingdom Hearts itself.

Mickey's perspective of this event also appears in Kingdom Hearts 0.2 Birth by Sleep: A fragmentary passage. The reason that Mickey loses his shirt is to partially keep the continuity of the original Kingdom Hearts that showed Mickey in his classic shirtless look.