List of post-release content in Splatoon

"Callie: We've got some update information to share! Marie: What is it? What is it?!"

- Squid Sisters

"Every week or two, new free content will be introduced in #Splatoon, including weapons, stages and gear."

- Nintendo of America's official Twitter

Post-release content is additional post-launch content released for Splatoon. Nintendo frequently released new weapons, stages, modes, features, and gear up until 2016-01-22. Usually, Nintendo announced the release date of upcoming content via their social media accounts. An Inkopolis News announcement introduces the new content when it has been released. Interestingly, most additional content was already in the files of Splatoon, but it was not unlocked by Nintendo until its official release. There have been 52 new weapons (counting the Rainmaker), 11 new stages and 3 new modes released. The content updates ended with the release of the Squid Sisters amiibo and their solo songs on July 6, 2016.

Squid Girl collaboration content
On August 5, Splatoon x Squid Girl collaboration content was released. Revealed on Masahiro Anbe's official Twitter, it features costumes from the manga/anime. Official art was also released with the tweet, and the following issue of Weekly Shōnen Champion also featured an ad to further confirm the crossover. Nintendo of America confirmed on Twitter before the update that the Squid Girl content would be coming to America.

Famitsu Squid Fashion Contest
In their May 20 issue, Famitsu offered a Splatoon costume design submission contest, with the winning entry being added to the game. The selected winner was The Festival Octopus Chef. The gear was added alongside the aforementioned Squid Girl gear in the August 5 update.

Super Smash Bros. for Nintendo 3DS/Wii U
Three Splatoon-themed costumes were added to Super Smash Bros. for 3DS and Super Smash Bros. for Wii U on 2015-06-14, two days before Nintendo's E3 Digital Event was held on 2015-06-16. They were announced in a Nintendo Direct hosted prior to Splatoon's launch.

Those who preordered Splatoon at GameStop received a free code for these costumes, although they couldn't be obtained until the official release.

The Splatoon-themed content includes a full outfit for Mii Fighters and a Trophy of an Inkling.