Mike MacKenzie, who represented the Highlands and Islands as a regional MSP between 2011 and 2016, said a lack of action from his former party on independence had forced his hand.
He said: “I have been disheartened at the insistence that progress on independence be kicked into the long grass until after Covid.”
The former candidate was sixth on the SNP’s list for the Highlands and Islands at the Holyrood election in May and required a complete collapse of SNP support in his region’s constituencies to be elected.
Alba’s policy on independence was that negotiations should begin between the Scottish Government and the UK the week after the election.
However the SNP, having won 64 seats in Holyrood out of 129 available, said a second independence referendum should take place after the Covid-19 panemic.
Announcing his defection, Mr MacKenzie said this approach constituted a “lack of progress”.
He said: “Like other former SNP activists I have looked on in astonishment at the lack of progress made by the Scottish Government since achieving its renewed mandate for a Referendum at the recent Scottish Election.
"The Highlands and Islands have suffered disproportionately from the economic impact of Covid with the tourism and hospitality sectors in particular taking a real hit during lockdown.
“I am looking forward to playing a full part in helping Alba develop the policies we need to tackle the unique challenges of the Highlands and Islands so that our people and communities can fully recover from the pandemic.
"We need to invest in transport infrastructure and broadband connections which will both retain and attract businesses to the Highlands, we must encourage young people to stay within their communities by making them attractive places to live and work and we need to sort out the chaos which exists across the CalMac network.
"Alba have the people, the ideas and the commitment to take Scotland forward.
"I am confident that ALBA's best days lie ahead and I am pleased to be joining them at this important time in Scotland's journey to Independence."