Ken Macintosh, Presiding Officer at Holyrood, wrote to officials in Shetland today to confirm that his choice of date was the end of August, following the former Scottish Executive Minister's resignation.
Mr Scott, a former Transport Minister in the Labour/Liberal Democrat coalition that governed at Holyrood from 1999 until 2007, confirmed last month that he would leave the Scottish Parliament to take up a position with the Scottish Rugby Union.
The Liberal Democrats will be heavy favourites to retain the seat that they have held since the inception of the modern Scottish Parliament twenty years ago, with the Northern Isles' reputation as a Liberal stronghold predating the modern party.
Orkney and Shetland, the UK Parliament seat which takes in the island, has been represented by a Liberal or Liberal Democrat since 1950.
The party has selected local councillor Beatrice Wishart as their candidate for the seat.
Labour have selected accountant Johan Adamson, and UKIP will be represented by Stuart Martin.
Two independents will challenge for the seat, while the SNP and Scottish Conservatives have yet to announce their candidates
The Shetland by-election will be the second constituency by-election in the current Scottish Parliament term, which started following the election of 2016.
The most recent Holyrood by-election saw the Tories retain Ettrick, Roxburgh and Berwickshire, after former MSP John Lamont stood down to take up a seat at Westminster.
Shetland is thought to be the second-smallest constituency by population, with around 23,000 residents.