The combined entity, called Walbrook PR, is set to have the second largest number of AIM-listed clients. It is currently at number four.
It is understood the merged company will have a turnover of around £1.5m a year.
Winningtons owners Tom Cooper and Paul Vann will join the Walbrook team.
The original Walbrook PR was founded six years ago and currently acts for 58 listed companies (mainly AIM) across a range of sectors including healthcare and technology. Winningtons, which has offices in Bristol, the City of London and affiliates in Leeds and Edinburgh, also specialises in financial PR, investor relations and IPO support.
Paul McManus, managing director of Walbrook PR, said: "Tom and Paul have developed a well-regarded financial PR business with a number of very strong working relationships, both in the City and with the network of professionals in and around the South West of England."
Cooper said: "We have found a team at Walbrook that shares our philosophy around client care and has ambitions to further grow an already successful practice."