In the 2021 Batman comic Legends of the Dark Knight #4, the front of the abandoned casino which Batman arrives to is based on the level Casinopolis from Sonic Adventure.

In the E3 1999 demo of Sonic Adventure, Knuckles had a very different voice. This was the same voice actor, but they re-recorded nearly all of his lines before the game's release. Surprisingly, voices for his entire story were included, even though the demo only had two stages for Knuckles.

The game's composer, Jun Senoue, initially went through and scrapped 8 different prototype iterations of the game's main theme song before settling on Open Your Heart.

Sonic Adventure had a limited edition release available two months before the release of the SEGA Dreamcast in North America. SEGA signed a deal with the game rental company Hollywood Video that let people rent a SEGA Dreamcast two months before the release of the console with a selection of games, one of those being Sonic Adventure Limited Edition. Although those copies were never meant to make it into the hands of the public, the copies that did make it to the public usually are incomplete or have a Hollywood Video sticker on the top of the disc.

Big the Cat was purposely created to move at a different pace and to break up the fast-paced action seen so far in the game. Fishing was chosen due to the fact that "he is well... a cat.".

The music for Red Mountain is a remix of "Crying World", the theme from Ristar. It also uses samples from Super Cat's "Ghetto Red Hot (Hip Hop Mix)".

Knuckles originally had an uppercut move but it was cut for unknown reasons.

In the HD port of Sonic Adventure DX: Director's Cut, starting Mission 12 as Knuckles reveals several posters from other Sonic games and media:
•Sonic X, the animated series
•Sonic Unleashed
•Sonic the Hedgehog 4
•Sonic the Hedgehog (2006)

During Sky Chase Act 2, Tails pilots the Tornado 2. The life bar, however, still shows the orginal Tornado 1 life bar. What's odd about this is that there is an unused graphic in the game's files that was going to be used for the Tornado 2, but goes entirely unused.

A leftover title screen from the Tokyo Game Show version of the game exists within the game's code.

Some objects were removed the game's original release, but were later discovered in the PC version of the game. The objects include an unused button in Lost World, a tumbleweed object in Windy Valley, a trash can object in Speed Highway, and an icicle object in the Icecap stage.

Six images can be found in the game's data that appear to be part of an old character select screen.

An unused Clock item icon can be found in the game's files. It's possible that it may have functioned the same way as "Chaos Control", or was a way of adding time to E-102 Gamma's levels, as they were all on a strict time limit.

There is an unused spring model that can be found in the game's files. It looks very similar to the springs used in Sonic Jam's Sonic World. Its texture appears to be used for the switches in the final game.

Several unused Tikal voice clips were discovered inside the game's files, including her talking about the Leon badnik, an unknown enemy called "Golah", and a clip referencing that Super Sonic was supposed to be usable in standard levels.

A prototype loading screen exists in the game's data. It was used in the Sonic Adventure '98 AutoDemo.

In the first cutscene of Sonic's story, Eggman is seen standing on top of building with a guard rail in front of him. This guard rail was removed, and the roof texture was changed in the DX version of the game.

As far back as 1999, composers in the Sonic series have been sampling from other tunes. The famous "Up and down and all around" and even the "Right!" that can be heard after it in the track Run Through the Speed Highway actually comes from James Brown's Super Bad.

In May 2013, Sonic Retro Researcher Orengefox posted his discovery of a Sonic Adventure prototype that is "without a shadow of a doubt the earliest Sonic Adventure prototype we've acquired". The build contains early Chao Garden music, unused graphics, a nameless Froggy, and several clues about early versions of Windy Valley amongst other things.

In preparation for Sonic Adventure, Sonic Team conducted a trip to Central and South America. This was done because none of the developers had ever seen environments like ruins and jungles, and the team wanted to make a realistic world for the game. Some of the pictures they had taken were used as textures for the game. Tails' Sand Hill sub game was even inspired by people boarding on sand dunes in Ica, Peru.