On the seventh floor opposite an enormous tapestry depicting Barnabas the Barmy attempting to train trolls for the ballet, the Room of Requirement appears only when someone is in need of it. To make it appear, one must walk past its hidden entrance three times while concentrating on what is needed. The room will then appear, outfitted with whatever is required. To the Hogwarts house-elves, it is also known as the Come and Go Room.

Dumbledore is the first to mention the room, noting that he discovered it at five-thirty in the morning, filled with chamber pots when he was trying to find a toilet. However, Dumbledore did not appear to know the Room's secrets. Dobby later told Harry of the Room in detail and admitted to frequently bringing Winky to the room to cure her bouts of Butterbeer-induced drunkenness, finding it full of antidotes and a "nice elf-sized bed." Filch was said to find cleaning supplies here when he had run out; when Fred and George needed a place to hide, it would appear as a broom cupboard. Trelawney also makes a habit of using it to hide her empty sherry bottles after she is sacked in Order of the Phoenix. It would seem that when one wishes to hide something it produces the same room for everyone: the Room of Hidden Things, which is full of many centuries worth of abandoned objects, such as broken furniture, books, and in one case a dead quintaped (for more information see Fantastic Beasts and Where to Find Them), which were presumably forgotten by their owners.

Harry learns of the room's abilities from Dobby in Order of the Phoenix, finding it the perfect location for his Dumbledore's Army meetings, during which it is filled with bookcases full of Defence Against the Dark Arts volumes, many different kinds of Dark Detectors, and a plethora of floor cushions for practising defensive spells. When the D.A. is betrayed, the room is left open, and Pansy Parkinson is able to retrieve the list of members of the organisation. In Half-Blood Prince Harry uses the Room of Hidden Things to stash his copy of Advanced Potion-Making, describing it as the size of a large cathedral and packed to overflowing with items hidden by Hogwarts inhabitants over the years, such as old potions, clothing, ruined furniture, an old tiara (which happens to be one of Voldemort's Horcruxes), or books which are "no doubt banned or graffitied or stolen." He later realises that Draco has been using the room in this state to hide and repair the Vanishing Cabinet to use it to smuggle Death Eaters into Hogwarts. Ironically, while Harry tries many times to get into the Room of Requirement to see what Draco is doing, the only time he succeeds to get into the room (and he is not thinking about Draco), he gains access to the room where Malfoy has been working.

In Deathly Hallows, the students who need a place to hide from the Carrows, two Death Eater professors, use the room. It is also revealed that the Room of Requirement's current version can change while still occupied, though should a completely different version be required (e.g. the Room of Hidden Things instead of DA Headquarters) the room must be empty. The Room can also answer to the desire of the wizard within the room, such as providing Harry with a whistle when he needed one during a Dumbledore's Army meeting, or creating a passage to the Hog's Head (as the room cannot produce food). Later, Ravenclaw's diadem is found to be one of Voldemort's Horcruxes and has been hidden in the Room of Hidden Things by Voldemort. Harry, Ron, and Hermione enter the Room, with Harry knowing that he must look for a place to hide things, and find the tiara; but they are ambushed by Draco, Crabbe and Goyle. The diadem is finally destroyed when Crabbe fills this version of the Room with what Hermione believes to have been Fiendfyre; a destructive magical fire. It is not known if the room continues to function after the events of Deathly Hallows; Ron expresses concern that it may have been ruined in all of its forms by the cursed fire.