This is one of the largest rooms in the mansion and owes its name to its function. It is here that foreign ambassadors present their credentials to the President of the Republic. The room is also used for the swearing-in of members of the government, for the presentation of awards and decorations, as well as for receptions.

This room was designed at the beginning of the twentieth century as a ballroom and constituted an important addition to the existing building. The work was assigned to Ziller by Princess Sophia in 1909. The extension was judged necessary because the main reception hall on the first floor was no longer adequate.

Ziller drew up two alternative designs for the roof of the room. The one which was finally chosen and realized was an innovative dome-shaped metal construction with a span of 10 metres and a length of 24 metres. The supervision of the work was carried out by the palace architect, Anastassios Metaxas.

The wealth of decoration in its interior is of special interest. Eight Ionic columns have been placed in pairs along the room. Their arrangement visually divides the room into three sections. In the middle section a small alcove is formed where there is a fireplace and exactly opposite there is a row of large windows. Above the alcove is the orchestra gallery.

The furniture which decorates this space and the large floor candlesticks originate in the French ateliers of the seventeenth, eighteenth and nineteenth centuries.

Leading to the Credentials' Lounge is a large oblong anteroom. Its walls are adorned with paintings by the well-known Greek artists, L. Geralis, P. Prosalentis, B. Hadzis and O. Fokas.