On the map "Museum D'Alfonsino" during it's Splatfest variant, two inkling statues can be heard making child-like giggling from opposite sides of the map. Strangely, the same sound clips can be heard upon defeating Koloktos in The Legend of Zelda: Skyward Sword.

Since no more Splatfests are being held for the game, this secret is inaccessible without hacking.

In 2015, Nintendo released a free DLC update to Splatoon, adding a costume from the Japanese manga and anime series, "Shinryaku! Ika Musume", or "Squid Girl", giving the player the titular character's outfit.

When you highlight one of the numbers on the password screen, a music note will play. If you follow a specific order, it is actually possible to play the final boss theme, also known as "Calamari Inkantation". The required number combination is:

767676765676767238767 676765672343212349432761

From June 1st until September 8th, 2015, Canadian Yogurty's and Yogen Fruz stores offered Splatoon-themed flavors and toppings to promote the game. The two flavors available were Squidsicle - a dairy-free mango popsicle sorbet - and Inkberry, a probiotic low-fat strawberry frozen yogurt. Mango or strawberry popping bubbles were also available as toppings.

On June 15th, several select Yogurty's locations also offered customers the chance to play Splatoon while at the store.

If you look inside of the entrance to the Battle Dojo while standing outside, a few Wii U controllers can be seen hanging on the wall. The controllers are a Wii U gamepad, three white Wii U pro controllers, and a black Wii U pro controller.

Sunken Scroll #17 states that the Squid Sisters, Callie and Marie, are actually cousins, not sisters.

Sunken Scroll #26 contains an image of a fossilized human surrounded by a Wii U GamePad, a box with the Wii U logo, a disc, and a Wii Remote strapped to a Wii U Pro Controller. It's text says "A 12,000-year-old fossil of a creature with an odd internal skeleton. Its small skull indicates the creature was likely primitive with little intelligence. This specimen appears to have been fossilized in the middle of some kind of ritual."

When Callie and Marie announce the map Moray Towers, Callie says "Rollers are super effective here!" and Callie says "Chargers are the very best, like no gun ever was!". Both quotes are references to Pokémon.

The Salt Spray Rig map was redesigned between it's reveal at Nintendo's Digital Event at E3 2014 and the game's retail release. The spawn location is lowered, and the area at the far top of the map is a lot bigger than in the final game. A number of other minor changes were also made.

According to Seita Inoue, during the development of Splatoon the characters started off as rabbits before becoming inklings.

"Well, a lot of it was because of design factors. Rabbits are white to begin with, so it would be easy to tell if they were inked. They've got long ears that would move when they did, so you could see them moving and when viewed from above, it would be easy to tell which direction they were facing by the direction of the ears."

Despite not being able to be seen during normal gameplay, the back of the amiibo box in Inkopolis is fully detailed just like a real amiibo box. Not only this, but the text written on the box is written in the same language as that used by the squids. The only way to see this is to glitch out of the Plaza.

The Octobrush is known in Japan as the Hokusai, referencing artist Katsushika Hokusai. The design is also based on traditional Japanese brushes. Interestingly, one of Hokusai's paintings was "The Dream of the Fisherman's Wife", depicting a woman having sexual intercourse with two octopuses, tying into the English name.

Sunken Scroll #23 contains a music sheet that, when played, is revealed to be a rendition of "Calamari Inkantation", the Squid Sisters song that plays during the 2nd phase of the final boss.

Prior to the game's release, Nintendo released a demo version of the title called "Splatoon: Global Testfire". The title screen for this release featured text in the bottom right corner that was written in the language used by the characters in-game. However, the text looks very similar to a racial slur in the English language. This screen was changed in the final release to show in-game locations instead. It is not known whether this change was always intended or whether it was made to remove the offending text.

In May of 2015, several days before the game's release, a truck filled with copies of the game and amiibos was hijacked and stolen on its way to the UK retailer warehouse GAME.

To promote the game, Nintendo had set-up on Santa Monica Pier an obstacle course inspired by the game where participants tried to complete it whilst getting covered in ink. A number of celebrities were also there attending the event including actor James Marsden, singer and model Alli Simpson, and actress Willow Shields.

According to Shigeru Miyamoto, Splatoon was almost made into a Mario game. Making it into a part of the Mario franchise would have assured sales, but it was eventually decided to keep the game as an independent and new IP.

The ink on the floor has varying levels of depth depending on how much ink has been layered on that area.

The audio department created a number of the ink sound effects by taking some slime and pounding at it.

The music used in the game's trailer includes a number of the staff who worked on the game shouting "Splatoon".