Which online service did Mario Kart DS use for online play?

Mario Kart DS used Nintendo Wi-Fi Connection for online play.

Nintendo Wi-Fi Connection was Nintendo’s free online multiplayer service for compatible Nintendo DS and Wii games. Mario Kart DS was the first Mario Kart title playable through the service, allowing players to race online with friends or opponents selected by region and skill level.

The game launched in 2005 and could support up to four players in an online match. This was separate from local wireless multiplayer, which connected nearby DS systems directly. Mario Kart DS also supported DS Download Play, allowing some multiplayer participation without every player owning a cartridge, but that was not the same as internet play.

Nintendo Wi-Fi Connection was later replaced by Nintendo Network on newer platforms and was discontinued on May 20, 2014. Nintendo Network and Nintendo Switch Online therefore should not be used as the answer for the original service. Miiverse was a social network and community feature, not Mario Kart DS’s online racing service.