He said: 'I'm very much looking forward to joining Edinburgh Rugby. I've always had at the back of my mind that I'd like to switch codes and now that this fantastic opportunity has come along I've jumped at the chance.
'I did play union in Sydney as a teenager but I'm not kidding myself that it will be anything other than a challenge to adapt. But I'm really looking forward to meeting the guys at Edinburgh and getting down to the business of trying to secure a place in the starting team.'
SRU Director of Rugby Ian McGeechan said today: 'The loss of Tom Philip with the knee injury he sustained on our summer tour of Australia has been a blow to Edinburgh Rugby.
'I made it clear at a meeting with the professional teams and the BT Premier Division 1 clubs back in August that, if finances permitted, we would look to strengthen our professional teams without putting undue pressure on other parts of the system. The revenues provided by our additional Abbey Autumn Test match against Australia mean that we are able to do so.
'We have been tracking Ben for just over one month and all the reports tell us he is a top quality player. Of course he will have to adapt from league to union but we are confident he will do so.'
MacDougall qualifies for Scotland through a paternal grandfather from Glasgow.
McGeechan added: 'We are obviously delighted that we have been able to recruit a talented player who we believe can not only do an excellent job for Edinburgh Rugby but could, if his performances merit it, potentially play for Scotland.'
Edinburgh Rugby head coach Frank Hadden said: 'I look forward to Ben joining us and adding to the competition for places in both our Celtic League and Heineken Cup starting teams.'
Prior to this season MacDougall had played 59 first grade games in NRL for Wests, Newcastle Knights and Manly Sea Eagles before joining Melbourne Storm in 2004.