In a statement written to his local party explaining his decision, Mr MacAskill MP for East Lothian said his move is ‘the best way to achieve our goal of independence.’
Mr MacAskill announced that his office will continue operating for constituents and he will continue to serve as an MP.
His move from SNP to Alba comes after former First Minister Alex Salmond announced his bid to return to Holyrood as leader of the new pro-independence party.
The MP voiced this morning in an opinion piece for The Scotsman that the new alba party is likely to worry the SNPs and the Scottish Greens.
Announcing his move, Mr MacAskill said: “I will be joining the newly formed Alba party to deliver that super majority for independence through the list vote which I believe is essential to achieving our Nation’s independence.
"Some members are doing likewise but it is very much a personal decision for us all and I hope you will understand my reasoning."
Mr MacAskill said his move comes as “it is essential to not only return an SNP Government, but to deliver a super majority for Independence” which Mr Salmond highlighted during the launch of the Alba Party yesterday.
He said the current SNP rules ‘don’t allow for a super majority’ for independence, adding: “It is for that reason, after considerable thought on my part, and after having been involved at various levels with the Party for over 40 years, I am relinquishing my SNP membership, to support this final push for our Nations' Independence.”
Ian Blakford, SNP Westminster Leader is calling for Mr MacAskill to resign his seat in the House of Commons, saying Mr MacAskill has been “an increasing embarrassment” for the SNP party and his move is “somewhat of a relief.”
Mr Blackford said: “After yesterday’s events this is the second least surprising news in Scottish politics.
"He has been an increasing embarrassment to many in the SNP and his departure is somewhat of a relief.
"That he is joining a party with serious questions to answer about its leader’s suitability for public office is no surprise.
"He should now resign his seat in the House of Commons to let a by-election take place immediately so the people of East Lothian can elect a new MP who will focus on their interests rather than self-interest.”
Mr MacAskill said in his email that the ‘social and economic harm’ predicted for Brexit under the UK Government is threatening Holyrood’s powers adding that, “the need for Independence has never been greater.”
He added: “Along with other colleagues in Alba I will work with the SNP group in Westminster in opposing the harm that is being done to our land by the UK.
"I will likewise continue to work for Paul McLennan’s election as the constituency MSP at the coming election in May.
“Whether in the SNP, Alba Party or Yes Movement, Independence is our shared goal. Let us go forward and deliver it.”