Glass is a clear, Ink-resistant surface; Ink that hits it will instantly dissolve away. Since Inklings cannot swim over it, it is more efficient to run across in humanoid form. Glass is also slightly transparent, if the player wants to take a look at what's under it.
Glass can be found on the walls and floors of several stages - Urchin Underpass has both platforms and walls made of glass in its side alleys. It sees a minor use in some other stages, including Arowana Mall, Camp Triggerfish and Port Mackerel.
Glass also appears during the final stage of The Rampaging Octowhirl!, the third boss battle in Octo Valley. Some of the rotating floor is destroyed, making it harder for Agent 3 to get the Octowhirl caught in their Ink. It is also used in some other missions, such as Propeller Lift Fortress.
Glass can be seen with stickers on them when looked at on the stage Urchin Underpass. If the stickers get looked at too closely, then the texture of the stickers becomes low resolution. The train in the main lobby is an example of a place where the glass is not transparent. In the main lobby, the player can see reflections from glass with a black filter over by Spyke. Any of the windows that have a sticker with an arrow pointing down with a squid shape is one of these when in the lobby.
Some glass in the old version of Urchin Underpass.