The Etruscan room is the final space in the parade of public rooms, and is one of the last surviving examples of the Adam Etruscan style. It is square in form with apsidal ends, Harris compares the space to the Countess of Derby’s dressing room at Derby House.

Unfortunately Adam’s painted Etruscan decoration does not survive. It is probable, as at Osterley, that the design was painted on to paper and laid over canvas which has since been stripped.
Adam’s chimneypiece, with its inlaid ornament painted in Etruscan shades of red, brown and black, survives in situ. Adam’s stucco work also remains, and this suggests the scheme was executed in full. The long panel paintings also survive, and Hussey and Oswald compare these to those included in the Etruscan room at Osterley.

Harris notes that there are no surviving designs for the ceiling, and it is possible that here, once again, Adam is adapting an earlier Wyatt ceiling. Ornamented in concentric circles, Etruscan elements such as linked winged sphinxes have been introduced, befitting the overall scheme.

A day-bed comparable to the state bed at Osterley was designed for the space, and there is some debate as to the use of the room, which is alternatively referred to as the third drawing room and the bedroom. Harris argues that the space is unlikely to have been used as a bedroom as this would not fit with the parade route and that Adam would not reserve his Etruscan style for a private space. However King notes that the early Adam Etruscan rooms, like that at 20 Portman Square, were often either dressing rooms or bedrooms as at Osterley and Apsley House. It is possible that the room served a semi public/private space befitting a house that wished to imitate the court and, with it, court etiquette. The inclusion of the final room as a bedroom or a private space to which perhaps only a small number may be admitted echoed the ritual of court procession and the grand levee. It is significant that the Etruscan room contains a concealed spiral staircase linking to a bedroom of the upper floor, suggesting that the room above was Lady Home's actual bedroom.

The Etruscan room at 20 Portman Square has recently undergone restoration, with Adam’s Etruscan painted detail carefully recreated using traditional materials.