The membership of the RIAS numbers approximately 5,200, which includes Honorary Fellows, Fellows, Members, Students, Affiliates and Retired Members. Any architect who is registered with the Architects' Registration Board (ARB) and is living and working in Scotland can apply to be a Member of the RIAS.

The Institute of Architects of Scotland was founded in 1840 by architects pre-eminent in the neo-classical revival in Scotland - William Burn, David Rhind, Robert Reid, James Gillespie Graham, William Playfair and Thomas Hamilton. the aim of the Institute was "cordial co-operation and frequent correspondence", leading to the establishment of a library, a museum, a drawings collection and a programme of occasional meetings.

That embryo body survived barely two years. It was refounded in December 1849 as the Architectural Institute of Scotland, with various classes of members in addition to architects - ironfounders, measurers and builders. It held regular meetings in Edinburgh, and by 1854, there was an equally vigorous group situated in Glasgow. By 1858, the Glasgow Group had formed itself into the Glasgow Architectural Association; and a junior body composed specifically for articled pupils and students, was founded as the Edinburgh Architectural Association. The Institute collected architectural drawings, and commissioned measured drawings of historic buildings, which it published in its Transactions. By 1873 the vigour of the two junior bodies in Glasgow and Edinburgh was such that the Architectural Institute was thought to be superfluous. Its functions were closed, and its assets and drawings transferred to the EAA for safe-keeping.

Yet the concept of a national architectural body for Scotland lingered, and was revived in 1897 and 1898, by which time three other architectural bodies - in Dundee, Aberdeen and Inverness had been established. No further progress was made until 1916, when those Architectural Societies met for dinner given in honour of Sir Robert Rowand Anderson, on the receipt of his RIBA Gold Medal. The outcome of that dinner was a series of meetings leading to the establishment of the Institute of Scottish Architects, funded by a generous gift of £10,500 from Anderson himself. The first annual Convention was held in Edinburgh in 1918 with Sir Rowand Anderson, as founder, the first President.

Premises for the Incorporation were founded by the donation by Sir Rowand Anderson of his town house in Rutland Square. Sir John Burnet, Anderson's successor as President, inaugurated a collection of busts of celebrated Scottish architects, and the series of portraits of Presidents. In 1922, a Royal Charter was granted to the body, under the title of the Incorporation of Architects in Scotland. In 1929, during the Presidency of Sir Robert Lorimer, a further Charter granted the title of "The Royal Incorporation of Architects in Scotland". In 1933, the sixth Chapter - the Stirling Society of Architects was founded.