Corporate partner Richard Masters and property partner David Bankier have moved from the Glasgow head office to run the Belfast practice.
They take projects assistant Gregor Hamilton with them and have poached corporate lawyer Kerry O'Hara from Belfast-based L'Estrange & Brett.
Fred Shedden, McGrigors' senior partner, says the office is looking for another partner and six more assistants.
The firm initially considered moving into Belfast because of an increase in PFI work across the whole of Ireland.
But Shedden claims that the decision to set up the new office was also due to the increase in other areas of work.
He says: "Our strategy is to go beyond that, including corporate work."
Bankier adds: "McGrigor Donald has been active in Northern Ireland since the early 1990s. It is partly in response to requests from our clients that we have taken the step of opening a Northern Ireland office."
The firms' existing clients in Northern Ireland include the Department of Environment for Northern Ireland, Northern Ireland Electricity and BCO Technologies