SonMario

SonMario is the slash ship between Mario from the Super Mario fandom and Sonic from the Sonic the Hedgehog fandom (and by extension, from the Super Smash Bros. fandom).

Mario & Sonic series
"Mario and Sonic have been respectful rivals since the early days of video games. In fact, for a long time they have been discussing the possibility of one day competing against each other. Now that they have been given the perfect opportunity to meet at the Olympic Games, we may finally learn who is actually faster, Mario or Sonic?"

- Shigeru Miyamoto, creator of the Mario series.

Mario & Sonic at the Olympic Games is the first official crossover between the world's two most popular video game characters. In this game, the two, along with other characters' from their franchises, participate in the Beijing 2008 Olympic Games. The two continued to participate in every Olympics until Rio 2016.

In the DS version of Mario & Sonic at the Olympic Winter Games and the 3DS version of Mario & Sonic at the London 2012 Olympic Games, Mario and Sonic team up to defeat Bowser and Eggman, who attempt to ruin the Olympics. In the Wii U version of Mario & Sonic at the Rio 2016 Olympic Games, the pair has a special victory animation where they fist bump and shake hands.

Super Smash Bros. series
Mario and Sonic have appeared together in the series since Sonic's introduction in Super Smash Bros. Brawl.

Behind the Scenes
At 1991, Sonic was created to provide Sega with a mascot to rival Nintendo's flagship character Mario. Nintendo and Sega were business rivals throughout the 1990s, though mostly during the competition of the Super Nintendo Entertainment System and the Sega Genesis / Mega Drive. As such, Nintendo and Sega often attempted to put each other down in their games. Their rivalry lasted until 2001 when Sega left the console manufacturing market with the discontinuation of their last console, the Dreamcast (in America and Europe) and became a third-party game developer to their former market rivals, including Nintendo.

Fanon
Although its popularity is overshadowed by popular rival ships such as Mareach and Sonamy, SonMario remains existing since the 1990s, that even it can be considered one of the earliest ships in Sonic's fandom thanks to fans creating fan art of them together on published video game magazines of that time. The popularity of this ship has increased considerably in 2007 thanks to Nintendo and Sega's announcement of the Mario & Sonic series, and therefore, the pairing become common in both fandoms.

Trivia

 * The ship is named Somari incorrectly, as this is the name used for an unlicensed port of Sonic the Hedgehog (1991) for Famicom.
 * For Mario's 20th anniversary, 2D artwork of Sonic wearing Mario's hat was made on Sonic Channel.
 * In the popular children's show Oha Suta which broadcasts at morning on Japanese television station TV Tokyo, Mario and Sonic were invited together to cheer to celebrate Sonic Adventure 2: Battle's launch and to promote Sonic Advance for Game Boy Advance.
 * Mario was invited to the Japanese Sonic the Hedgehog's 25th Anniversary Party to promote Mario & Sonic at the Rio 2016 Olympic Games's launch and to celebrate his rival's birthday.
 * At Fund The Charity Room 2016, Jason Griffith —Sonic's former English voice actor— has flirted with Mario (who was performed by YouTuber SomecallmeJohnny at the moment) jokingly in his Sonic character.