UNION is the opening theme song of the 2018 anime series SSSS.GRIDMAN, a modern reimagining of Tsuburaya Productions' classic tokusatsu series Gridman the Hyper Agent, produced in collaboration with studio Trigger. The track was composed, written, and performed by OxT, the musical duo of Masayoshi Oishi (オーイシマサヨシ) and Tom-H@ck, and captures the energetic, heroic tone of the series with driving rock instrumentation and an anthemic chorus. UNION made its rhythm-game debut in maimai DX (でらっくす) on July 19, 2019, as part of a limited-time SSSS.GRIDMAN collaboration area. During the event window (July 19 through September 19, 2019), players needed to progress 10 meters through the dedicated area to unlock the song; it became available by default from October 4, 2019 onward. The song was eventually removed from the maimai DX arcade series on March 24, 2022, and is no longer playable in current versions. The MASTER chart, designed by はっぴー (Happy), is noted for embedding SOS-shaped Touch note formations in sync with the song's lyrics, a thematic callback to the narrative of SSSS.GRIDMAN. The Expert chart was designed by Jack. Both charts received difficulty level adjustments during the game's lifecycle: Expert was originally Lv 9+ at launch before being revised upward to Lv 10 from でらっくす PLUS onward, and Master was upgraded from Lv 11 to Lv 11+. The maimai version uses the TV-size edit of UNION. The song also appears in SEGA's CHUNITHM CRYSTAL (where the artist credit reads OxT「SSSS.GRIDMAN」) using a different cut of the full-length recording.