Quill, set up in 2000, is a specialist financial services shop led by founder Fiona Harris. The agency will continue to operate under the Quill name and will relocate to Buchanan's offices.
The terms of the deal were undisclosed.
Buchanan chairman Richard Oldworth said: 'Its expertise in investment and wealth management is the perfect complement to our financial services offer, and we see cross-referral potential between our companies.'
Buchanan has traditionally focused on financial PR and investor relations, building a strong reputation in the natural resources sector. Buchanan is a leading financial PR firm for AIM clients, but is keen to target bigger businesses.
Buchanan is led by MD Bobby Morse, who took over day-to-day running from Oldworth in March 2010 when the latter became chairman.
Quill's client roster includes BNP Paribas Investment Partners, Cazenove Capital Management and M&G.