Perth law firm Jackson McDonald has confirmed it will absorb boutique firm McKenzie Moncrieff with the two operations merging under the former's banner from September 1.

The move, which was foreshadowed by WestBusiness last month, follows at least six months of talks between Jackson McDonald and several firms in the WA commercial and corporate space.

The merger will create a firm with 32 partners and 120 professional staff reinforcing Jackson McDonald's status as one of WA's biggest law firms.

Jackson McDonald chairman of partners Graham Goerke described the merger as a "natural fit".

"McKenzie Moncrieff has established a solid reputation for their expertise in the corporate advisory area and mergers and acquisitions in the mining and other sectors, which are major growth areas for our clients," he said.

"They also have a high profile in complex corporate transactions and reconstructions.

"They will bring valuable additional capacity to Jackson McDonald through and beyond the current mining and energy boom."

McKenzie Moncrieff executive director Rob McKenzie said the firm had been approached by several international and national firms before deciding to proceed with Jackson McDonald.

"It makes sense to offer our clients a bigger team, with a broader range of skills, but with the same personal service from top lawyers," he said.

"In particular, joining Jackson McDonald will expand our capacity to deliver in areas such as workplace relations, IP and taxation."