Ink is a liquid containing dye(s) or natural colouring(s). When it is used, it produces images and texts on various surfaces and media.
Ink is used in conjunction with quills to write on parchments by wizards and witches in the wizarding world in place of the pens and pencils in the Muggle world. There are several types of inks, such as Azul Marino ink, various forms and types of invisible ink, Disappearing Ink, glow-in-the-dark ink, acidic ink, Self-Correcting Ink, Colour-Change Ink and Everlasting Ink. Harry Potter seemed to prefer a scarlet-coloured ink.
As part of its abilities as a Horcrux, Tom Riddle's diary could absorb ink, and could interpret any messages written in it when it absorbed them. It could also respond to these messages in ink, though it would reabsorb the ink used after a few moments. When it was destroyed by Harry Potter in 1993, it leaked a large quantity of ink, similarly to how a human would bleed.
Behind the scenes
- In "The Black Lake" level of LEGO Harry Potter: Years 1-4, as Harry Potter and Cedric Diggory swim through underwater tunnels, the Giant Squid cuts through the rock and releases ink, forcing Harry and Cedric to use Lumos Solem in order to find their way to the end of the tunnels.
- Even though it was not on the list of required equipment that was sent out each year, it is presumed that students would have had to buy ink and parchment (enough to last them until the next holidays).
- In 1995, Dolores Umbridge had a quill which she made students use in detention but it did not require any ink as it used the user's own blood and etched whatever was written into the skin.
- Magical Regulation 572 is a rule issued by the British Ministry of Magic stating that the only ink to be used in examinations is Azul Marino ink. 
- In the wizarding world, ink is not permanent and smudges can easily be erased by using the Smudge-removing spell, with the possible exception of Everlasting Ink.
- Though Harry Potter uses scarlet-coloured ink in the novels, he is seen to use inks of other colours, such as blue or black, in the films.
- The feathers of the Augurey repel ink, thus making them using as quills.
- Harry Potter and the Philosopher's Stone (First appearance)
- Harry Potter and the Philosopher's Stone (video game) (Mentioned only)
- Harry Potter and the Chamber of Secrets
- Harry Potter and the Chamber of Secrets (film)
- Harry Potter and the Prisoner of Azkaban
- Harry Potter and the Goblet of Fire
- Harry Potter and the Order of the Phoenix
- Harry Potter and the Order of the Phoenix (film) (Mentioned only)
- Harry Potter and the Half-Blood Prince
- Harry Potter and the Deathly Hallows
- LEGO Harry Potter: Building the Magical World (Ink bottles)
- LEGO Harry Potter: Years 1-4
- Harry Potter Trading Card Game
- Harry Potter LEGO Sets (Ink bottles)