The deal has taken 18 months to complete.
Grahame Underwood, a partner at London-based Watkins Gray, said Group Goetz would bring its US clients over to the joint venture's UK offices.
He said: "One of the reasons that Group Goetz wanted to make the move was that its clients have been finding it difficult to get offices in Europe fitted out to the standard of their US offices."
He added that Group Goetz's clients, such as oil giant Exxon Mobil, do not want to source out all their office work, and are keen to have an interior design team on retainer.
GGA Watkins Gray's UK offices are based in London and Leeds, with associated offices in Manchester and Belfast. The company's US offices are in Washington DC, Reston, Virginia, and Bogotá, Colombia.