England international Johnstone departs having made 167 appearances for the club since arriving from Manchester United in July 2016.
The goalkeeper, who leaves as a free agent after his contract expired, had been offered a new deal to remain at The Hawthorns.
But numerous clubs in the Premier League are understood to have been chasing his signature with Tottenham, Southampton and West Ham all linked.
Albion have also confirmed they will not take up their option to extend Romaine Sawyers’ contract by a further year.
The Express & Star revealed last week that the midfielder's time at The Hawthorns had come to an end.
Sawyers was a key part of the Baggies side that won promotion under Slaven Bilic.
But he was deemed surplus to requirements under former boss Valerien Ismael and spent last season on loan at Stoke.
Albion's released and retained list also states Andy Carroll has left the club – something Steve Bruce confirmed last month.
The Baggies have offered new deals to five players from the academy – including Reyes Cleary.
The Express & Star understands striker Cleary has told the Baggies he won't be signing a professional contract with the club.
By offering the 18-year-old a deal, though, Albion will be entitled to compensation for the forward who scored 13 goals in 11 appearance for the under-18s last season and netted seven times in 15 outings for the under-23s.
Albion have also offered new deals to youngsters Ted Cann, Josh Shaw, Samuel Okoka and Reece Hall.
But Jamie Soule, Owen Windsor, Aurio Teixeira, Kevin Joshua, Zak Delaney, Mark Chidi, MacKenzie Lamb, Leon MacHisa and Daniel Ngoma have all been released.
A statement from the club said: "The club would like to thank all departing players for their efforts during their time at The Hawthorns and we wish them every success in the future."