A Holyrood by-election is to be held on the same day as the UK's snap general election on 8 June.
John Lamont quit the Ettrick, Roxburgh and Berwickshire constituency seat to fight for a place at Westminster, hoping to unseat the SNP's Calum Kerr.
The Conservative MSP formally resigned from the seat as of 13:00 on Thursday.
The by-election means residents in the area will vote on their local council, their Holyrood representative and their MP all inside five weeks.
Selection procedures for the Holyrood by-election have yet to get under way.
Presiding Officer Ken Macintosh has written to Tracey Logan, chief executive of Scottish Borders Council and returning officer for the elections, to confirm his choice of date and to ask for arrangements for the by-election to be put in place.
Mr Lamont is seeking to unseat the SNP's Calum Kerr in Berwickshire, Roxburgh and Selkirk, Scotland's most marginal Westminster constituency. Mr Kerr gained the seat from Lib Dem former Scottish Secretary Michael Moore in 2015 by a margin of 328 votes.
The Lib Dems have put forward Catriona Bhatia, currently deputy leader of the council, as their candidate for the Westminster poll.