The Locked Room (original Swedish title: Det slutna rummet) is a mystery novel by Swedish writers Maj Sjöwall and Per Wahlöö, published in 1972. It is part of their detective series revolving around Martin Beck and his team.

The story begins with Beck recovered after his injury sustained at the end of the previous book and now returning to the National Police Bureau; he discovers that the police force still is amateurish and unprofessional for the most part, which he was accustomed to previously. Patrolmen have found a highly decomposed corpse (as it turns out later, a murdered man--found in a locked from inside apartment. An incompetent detective, Bo Zachrisson, had the case shelved as suicide. No one was much disturbed by the fact that the weapon had not been found in the apartment.

The novel now has two storylines: the locked room investigation and the bank robbery specialists.

The latter take by accident the petty criminal Mauritzon, a mate of Malmström and Mohrén, and accuse him of a bank robbery in which a bank customer was shot. At the same time Martin Beck solves the locked room mystery- the gun was shot from outside by Maurtizon, when the window was open. The window had fallen so that the latch had closed by itself.

Since the evidence collected by Martin Beck is insufficient, Mauritzon is acquitted of murder; but for the bank robbery, of which he is innocent, but whose evidence he cannot refute, he is sentenced to manslaughter with lifetime imprisonment.

The bank robbers pursued by Attorney Olsson manage to fool the police (the robbery does not occur in Stockholm, but in Malmö instead).

During the investigation, Beck, in a depressive phase of his life, meets a woman named Rhea Nielsen and he finds new courage which in the following novels plays an important role.