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

Downloadable content, or DLC, is additional post-launch content released for Splatoon. Nintendo frequently released new weapons, stages, modes, features, and gear up until January 22nd 2016. 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 is already in Splatoon, waiting to be unlocked by Nintendo. There have been 52 new weapons (counting the Rainmaker), 11 new stages and 3 new modes released thus far.

Upcoming DLC weapons
No currently upcoming weapons.

Other
Nintendo stated that the August update was not the end of the free content coming to Splatoon, and that more content will continually roll out through the rest of the summer and into autumn. The Downloadable Content finished with the release of the Custom Hydra Splatling on January 16th, 2016.

Squid Girl collaboration DLC
On August 5th, a Splatoon x Squid Girl DLC was released. Revealed on Masahiro Anbe's official Twitter, the DLC featured 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 DLC would be coming to America.

Famitsu Squid Fashion Contest
In their May 20th 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 DLC was added alongside the aforementioned Squid Girl DLC in the August 5th 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 June 14th 2015, two days before Nintendo's E3 Digital Event was held on June 16th 2015. 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, though they couldn't be obtained until the official release.

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