That will see turnover at the enlarged firm rise by around 10 per cent to somewhere above £23 million.
Bird Semple, which can trace its roots back to 1845 and has its main office on Blytheswood Square in Glasgow, has a turnover of around £2.1 million.
At the moment it provides a range of specialist services including wills and succession, asset protection, wealth management and personal taxation.
Bird Semple's 35 employees, including six partners as well as other lawyers and support staff, will relocate to Harper Macleod's Ca'd'oro building on Gordon Street in Glasgow on September 1.
Once there they will combine with the existing private client team. Harper Macleod has taken steps to make room for the additional numbers by increasing the office space at its headquarters by around 15 per cent as part of a refurbishment programme.
It is thought likely the Bird Semple brand name will be retained.
Harper Macleod, the legal adviser for the Glasgow 2014 Commonwealth Games, believes it will have one of the largest private client practices in Scotland from September, with more than 50 dedicated staff.
The firm hopes to grow its private client operation from its other locations, in Edinburgh and Inverness, on the back of the partnership with Bird Semple.
Lorne Crerar, chairman of Harper Macleod, said: "Bird Semple helps us to extend our private client offering in wealth management, age management and there is a tax angle which we haven't had before.
"It really does extend our reach and expertise in that area of work.
"There is a strong business case for Harper Macleod, and Bird Semple [has] very nice people, who I think we will get on well with."
Mr Crerar said talks about the deal have been going on for around nine months and stemmed from Harper Macleod's desire to extend its private client practice.
He said: "The opportunity that existed for our business to be continually sustainable was to get additional expertise. Bird Semple stood out as being the firm we could most work with.
"The legal business is undergoing significant seismic shift and finding two good partners to come together is a very difficult exercise. We have managed it with them and it is very positive."
Frank Fletcher, managing partner of Bird Semple, said: "This is a positive and exciting development for our clients, partners and staff. The last few years have ushered in a period of significant change in the legal profession. Looking to the future, we believe the interests of our clients are best served by combining our own specialist expertise with that of colleagues in another strong and successful firm in Scotland."
Mr Crerar confirmed around half of the additional space the firm has added in Glasgow would be taken up once Bird Semple move in, while the Inverness office has also been extended. He said: "We are pretty sure the way things are going at the moment we will continue to grow."
The firm is also looking at ways to capitalise on its involvement in the Commonwealth Games.
Mr Crerar added: "The Commonwealth Games has been an enormous opportunity grasped by the firm and if we can just make the most of the legacy then again I think we will see significant growth."
Bird Semple's commercial and corporate practice joined with what was to become DLA in 2000.
In the 12 months to March 31, 2013, Harper Macleod saw its turnover rise from £19.2m to more than £21m with operating profit going from £6.3m to almost £7.7m.