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 releases new weapons, stages, modes, features, and gear. Usually, Nintendo announces the release date of upcoming content via their social media accounts. An Inkopolis News announcement introduces the new content when it is released. Interestingly, most additional content is already in Splatoon, waiting to be unlocked by Nintendo. There have been 16 new weapons, 5 new stages, 1 new mode category, and 2 new modes released thus far.

August Update Event
On August 5th, 2015 ❌/August 6th, 2015, a "huge matchmaking update" will be released. The following is part of the update:
 * Private Battle - Players are able to create custom online or LAN lobbies of 2-8 players and choose the stage and the mode that will be used
 * Squad Battle - Players are able to form squads of friends and play online Ranked Battles with other squads
 * Level cap increase - New cap is Level 50 (from Level 20)
 * Two new Ranks - S and S+ (both higher than A+, the previous max Rank)
 * Change in the calculation method for Rank Points based on the results in Ranked Battle
 * Users will be sent back to the Battle Lobby when their ranks change in Ranked Battle
 * Splatfest winning percentage will be multiplied by 4 rather than 2
 * Stage changes
 * Urchin Underpass - Overall layout has been adjusted in all modes
 * Saltspray Rig - Adjusted layout in Tower Control
 * Arowana Mall - Adjusted layout in Splat Zones
 * Kelp Dome - Adjusted layout in Splat Zones and Tower Control
 * Moray Towers - Adjusted layout in Splat Zones and Tower Control
 * Special Weapon/Special Gauge changes
 * Points needed to use Inkzooka: 180 -> 220
 * Points needed to use Echolocator: 180 -> 200
 * Points needed to use Killer Wail: 180 -> 160
 * Minor adjustments/corrections
 * Slosher weapon type - Bucket-like weapons that hurl ink straight ahead with a high range
 * Bloblobber
 * Splatling weapon type - Guns capable of firing many small blasts of ink at once in rapid succession after being charged up
 * Heavy Splatling
 * Over 40 new pieces of gear
 * Unnamed leather jacket (Clothing)
 * Unnamed sushi-chef costume (Famitsu Squid Fashion Contest)
 * Splatoon x Squid Girl crossover gear (unnamed)
 * Strapping Whites (Shoes)
 * Logo Aloha Shirt (Clothing)
 * Unnamed straw hat (Headgear)
 * New music from new bands, Hightide Era and ABXY
 * Hooked - Hightide Era
 * Friend List - ABXY

Nintendo has stated that the August update is 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.

Squid Girl DLC
Sometime in August, a Splatoon x Squid Girl crossover DLC is due. Revealed on Masahiro Anbe's official Twitter, the DLC is to feature 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. No word has been made on the DLC leaving Japan.

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 will be added alongside the aforementioned Squid Girl DLC.

Confirmed DLC leaks
Some leaks for upcoming DLC have been confirmed.

Datamine DLC leak
NWPlayer123 datamined Splatoon and found files for upcoming DLC weapons, stages, and modes.

Prima Strategy Guide "Coming Soon" section
The Prima Strategy Guide for Splatoon includes a "Coming Soon" section which shows off weapon and stage DLC. There are also two images of Tower Control. All of the content shown has been released.

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.

On Friday June 5th 2015, Masahiro Sakurai, the creator and director of the Kirby and Super Smash Bros. series, officially announced on a Miiverse post that the DLC costumes would be released.

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.