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.

Brenda Wardle - who denies the charges - is accused of pretending to be a practising attorney and offering to represent a convicted murderer.
She appeared at Port Elizabeth Commercial Crimes Court on Monday, South African media report.
She featured as an expert on the BBC during the 2014 Pistorius trial.
Ms Wardle, 56, also boasts of appearing on Sky News, Fox News and a number of South African broadcasters on her social media profiles.
She has published a book called To Kill A Fragile Rose: The State's Case Against Oscar Leonard Carl Pistorius.
According to local media, these allegations date back to 2009, when she offered to represent a convicted murderer.
The family accuse her of charging around 500,000 rand (£28,600; $36,800) for her services over the next four years.
Ms Wardle will represent herself, against the advice of the magistrate, The Herald reports.
She was refused bail after failing to arrive at court on several occasions, IOL News said.
Ms Wardle will next appear in court on 27 July.
A Holyrood by-election is to be held on the same day as the UK's snap general election on 8 June.
[NEXT_CONCEPT]
A woman who acted as a legal analyst for the media during the Oscar Pistorius trial has appeared in court charged with fraud.