Pillsbury Winthrop Shaw Pitman's London office is targeting the AIM market following a raid on Osborne Clarke.
Osbornes partner Simon Fielder and assistant Caroline Grange specialise in flotations on AIM and the main market, particularly advising North American issuers.
Fielder said: "We're looking forward to the chance to be able to work with the US corporate group in augmenting our existing base of Canadian clients with US companies looking at M&A or financing options in Europe."
Pillsbury has struggled to assert itself in London after a series of partner departures during the past three years left it with just a token presence in the capital.
Pillsbury hopes that Fielder's technology, media and telecoms client base will complement that of the firm's US West Coast operation. His clients include News International, Friends Reunited, online video game company Fun technologies and IT consulting services company Spearhead.
He has been a partner at Osbornes since 1999 and joins Pillsbury as a partner. Granger joins as an associate.