At Tulchan she will take on senior consulting duties and work alongside founding partner, Andrew Grant.
Hamilton said: 'I didn't want to be a journalist forever. I've reached a point where I wanted to try something different.'
She is the third section editor to leave the Sunday Times for the PR industry in the past 16 months.
Last year political editor Michael Prescott joined Weber Shandwick as a vice-chairman (PR Week, 29 June 2001). And in 2000, business editor Andrew Lorenz left to join Financial Dynamics as a director (PRWeek, 6 October 2000).
Grant said he had been trying to entice Hamilton onto the payroll 'for some time.' He added: 'She had previously said that she didn't want to leave journalism. I'm delighted that she now feels the time is right.'
Hamilton, the first high-profile journalist hire at Tulchan, will work with existing clients. These include Boots and ScottishPower.
Hamilton joined the Sunday Times in 1992 as a business reporter. She left for a year during the mid-1990s to join the Sunday and Daily Express as City editor, before moving back.
The decision to hire Hamilton comes just three months after the departure of Maria Darby from Tulchan (PRWeek, 16 November 2001).