Placket sold down his stake in the business, which had stood at 117,807 shares worth £1.82 million at a share price of £15.49, up 90% over the last three years versus a FTSE 250 return of 48%.
The shares are held in his BlackRock UK Emerging Companies Hedge fund, which he runs alongside long-time co-manager Ralph Cox.
The private client investment management sector has been on a tear over the past year as asset prices have risen hard. In an interim statement last month Rathbones reported assets under management up 12% since 2012, to a record £20.1 billion.
Broker Canaccord Genuity reduced its recommendation on the company from buy to hold following the announcement. Numis has a reduce recommendation while Shore Capital rates it a hold.