The merged practice will boast 25 partners and over 250 staff and will trade under the Blackadders banner.
Paul Devine and Colin McCulloch, directors at Boyle Shaughnessy, will become partners with Blackadders.
Johnston Clark, Blackadders managing partner, said: “We are delighted to be strengthening Blackadders by joining forces with Boyle Shaughnessy, a well-known and respected law firm in Glasgow.
“This will give the firm a strong presence in the major centres of Scotland, offering a full range of services from our Aberdeen, Dundee, Edinburgh, Perth and Glasgow offices.
"Our growth into a national firm will create increased strength and depth of client service.”
This latest move follows the recent successful mergers between Blackadders and the Edinburgh practice of Colin Blaikie and the long-established Perth practice Condies.