Johnny Chatterton, who has led high-profile online campaigns in support of Burmese democracy and against bank charges, will join 38 Degrees at the end of this month.
Headed by Body Shop International co-founder Gordon Roddick, 38 Degrees launched in May this year, touting itself as the first website to bring direct online campaigning to the UK.
One of its current campaigns, 'holiday watch', scrutinises MPs' movements, while another opposes bankers' bonuses.
'We think there are thousands of people out there who feel that their voices aren't heard by decision-makers,' said Chatterton, adding that 10,000 people had signed up to 38 Degrees in the seven weeks since its launch.
'The whole premise of 38 Degrees is people and change, whether that's political change or targeted at companies,' said Chatterton. 'We'll be working with a number of single-issue campaign groups, engaging our supporters with these issues to scale up their campaigns.'
Left-leaning political advocacy group MoveOn.org has taken a lead on a number of issues in the US, organising campaigns against the Iraq War and in support of healthcare reform.
New media strategist Mark Hanson said: '38 degrees has huge potential, because what we've not been able to do in this country is mobilise people in the way that political parties could ten to 15 years ago. Johnny Chatterton is one of the few people in the UK who has experience of doing exactly that.'