Acquisition makes multidisciplinary consultant the second biggest architect in the UK.
Ruddle Wilkinson will become Capita Ruddle Wilkinson and will sit alongside Capita Percy Thomas to make a practice of more than 200 qualified architects and an expected turnover of £30m.
Ruddle Wilkinson is framework design consultant to BAA, Tube Lines, Network Rail, the London Borough of Barnet and Hatfield & Welwyn Council. It also has preferred architect status with leading clients such as IKEA, Marks & Spencer, Royal Bank of Scotland, Capital and Regional and Redrow Homes.
The acquisition is the first of a series in what is expected to be a summer of expansion at Capita Symonds. The firm refused to reveal the cost of the acquisition.
Rob Firth, head of architecture at Capita Percy Thomas, said: "Ruddle Wilkinson is a highly respected practice with a wide range of blue chip clients in key sectors where, historically, we have not been represented.
"This move brings us over 100 well motivated and highly skilled staff, led by a team of talented directors that are specialists in their field. Its portfolio of work is a great fit with our existing offer, and its offices in London and Peterborough reinforce our presence in London and the Midlands."
Norman Taylor, who was managing director of Ruddle Wilkinson and now director of architecture at Capita Ruddle Wilkinson, added: "This acquisition represents the most significant and exciting development in the 120-year history of Ruddle Wilkinson.
"Having grown by over 300% over the past four years, becoming part of Capita Symonds will enable us to deliver our ambitious business plans while providing our staff and clients with the security that the backing of a major multidisciplinary consultancy and its FTSE 100 parent can bring.