On the pre-release European box art, the character "Beat" is shown with 6 digits on one of his hands. This image of beat was used in several other pieces of promotional material.

One of the songs, "Super Brothers" by the band "Guitar Vader", is very obviously about Super Mario Bros. This is particuarly unusual, as Jet Set Radio is a SEGA developed game and published from the year 2000, when SEGA and Nintendo were still competitors.
"Come We're going to rescue Peach. We're super boys. 1234567, we like mushrooms."

In the HD version there is an achievement called "When You're a Jet", which is a reference to West Side Story.

The song "Let Mom Sleep" contains a sample from an old British sitcom called Hancock's Half-Hour.

The song "Rock It On!" contains two main samples. The first "Rock that shit, homie!" comes from the song "Terminator X Speaks With His Hands" by Public Enemy.

The other, "Tune in. Turn on. Drop out." is from the Human Be-In Speech by Dr. Timothy Leary.

In the game's data are some early versions of posters located in Benten-Cho. These posters included a parody of a Batman movie poster with the name of DARKMAN. This poster possibly went unused due to 'Darkman' already being copyrighted. Another sets of posters were two porno movie posters that can also be found in the files DEMO17 through DEMO20. They were meant to be displayed in Benten-cho's center circle.