Sonic Underground is an animated series co-produced by TF1, DiC Entertainment (which was owned by Disney at the time) and Les Studios Tex. It is the third Sonic the Hedgehog animated series, and also the last one produced by DiC; it follows a main plot separate from all other Sonic the Hedgehog media, where Sonic had two siblings, Sonia and Manic, that were collectively part of a royal family who were forced to separate from their mother, Queen Aleena, upon Robotnik's takeover of Mobius due to a prophecy told by the Oracle of Delphius. It first premiered in France on 6 January 1999 on TF1, and then premiered in the United Kingdom on 2 May 1999 on ITV and finally in the United States in syndication on 30 August 1999 and ended on 22 October 1999.

Sonic Underground first premiered in France on 6 January 1999 and aired on TF1 on the TF! Jeunesse block on Wednesdays and Sundays.  The show later premiered in English in the UK on 2 May 1999 and aired on ITV1 on Sunday mornings. ITV1 aired the first 18 episodes in the original UK run before cancelling the show.  In the United States, Sonic Underground premiered on 30 August 1999 (one day after the show was cancelled in the UK) and aired on the Bohbot Kids Network syndication block on weekday mornings at 6:00 AM Central Time. It ran for one season from 30 August 1999 to 22 October 1999. On the Sci-Fi Channel (formally part of the Bohbot Kids Network), the episodes that were supposed to air on Fridays were skipped because Double Dragon and later King Arthur and the Knights of Justice were airing instead, therefore only airing 32 of the 40 Sonic Underground episodes. It also aired in Canada on Teletoon.  Sonic Underground was later rerun. In the United Kingdom in 2005, Sonic Underground aired on Pop, which has also aired Adventures of Sonic the Hedgehog on its sister station Kix!. Between 2005 and 2006, it aired on ITV2 on the Action Stations! block. On 19 January 2006, CBS announced a multi-year deal with DIC to broadcast some of their shows on the "CBS's Secret Saturday Morning Slumber Party" segment, including Sonic Underground. In 2009, reruns of Sonic Underground aired on Firestone Communications' Sorpresa, a Hispanic children's station (Channel 850 on Time Warner Cable) in the United States, were broadcast audio-dubbed Latin Spanish episodes. In 2011, the show was broadcast by KidsCo. Re-runs of the series aired on Disney XD starting 11 June 2012 and ending on 14 December 2012. This makes it the second time a Sonic the Hedgehog cartoon airs on a Disney-themed channel with the first being Adventures of Sonic the Hedgehog on Toon Disney.  In South Africa, Fox Crime had originally planned to air Sonic Underground on 21 August 2015 as Case File Toon service of DStv, TopTV, SpaceTV, My.T and ZAP, but has been put on hold.

When did the series first air?
6 January 1999