Moores Rowland has eight partners and will be integrated into Mazars, which has 20 financial planners and a total advisory team of 35 in the UK.
Moores Rowland specialises in advising companies listed on the Alternative Investment Market and will pass referral business on to Mazars Financial Planning.
The merger will see Mazars increasing its turnover in the UK to £90m.
Mazars operates from 18 offices throughout England and Scotland but Moores Rowland will continue to be London-orientated.
The merger will enable Mazars to expand its service offering to cover audit, transaction services, tax, corporate finance and regulatory compliance.
Mazars Financial Planning chief executive Paul Willans says: “This merger will be of huge commercial benefit to our London region financial planning team.”