A State Room in a large European mansion, is usually one of a suite of very grand rooms which were designed to impress, they were the most luxurious in the house and contained the finest works of art. State rooms are usually only found in the houses of the upper echelons of the aristocracy, those who were likely to entertain a head of state. They were generally to accommodate and entertain distinguished guests, especially a monarch and or a royal consort, or other high ranking aristocrats and state officials, hence the name.

In England in particular state rooms in country houses were seldom used. The owner of the house and his family actually lived in the "second best" apartments in the house. There was usually an odd number of state rooms for the following reason: At the centre of the facade, the largest and most lavish room, (for example at Wilton House the famed Double Cube Room) this was a gathering place for the court of the honoured guest. Leading symmetrically from the centre room on either side were often one or two suites of smaller, but still very grand state rooms, for the sole use of the occupant of the final room at each end of the facade - the state bedroom. The smaller (but still huge) rooms in between would be used for private audiences, a withdrawing room and a dressing room. The smaller rooms were solely part of the bedroom suite and not for public use.

From the early 18th century, as aristocratic lifestyles slowly became less formal, there was a move on the one hand to increase the number of shared living rooms in a large house and to give them more specialised functions (music rooms and billiard rooms for example) and on the other hand to make bedroom suites more private. In houses from earlier than around 1720 which survived without major structural alteration, the state rooms sometimes became a meaningless succession of drawing rooms and the original intention was lost. This is certainly true at Wilton House, Blenheim Palace, and Castle Howard. On the other hand there were a few houses, and royal palaces, most of them exceptionally large, which were laid out in such a way that the state rooms could be left in their original form, while other rooms were converted or added to meet the new needs of the 18th and 19th centuries, or where funds were available to simply add on extra wings for the new purposes. Examples of such residences with surviving state suites which have never really changed their function include Windsor Castle, Chatsworth House, and Boughton House .

One board a ship the term 'state room' defines a superior first class cabin.