But while both sides are keen on the shot-stopper, neither are willing to meet the Baggies £20million asking price.
Johnstone was in outstanding form for Albion last season with his performances earning him a spot in Gareth Southgate’s England squad for the European Championships.
But those displays have also led to speculation he will leave The Hawthorns with Manchester United, Tottenham and Leeds among the other clubs to have been linked with him.
With just 12 months remaining on his contract, Albion know if they don’t sell Johnstone this summer they could lose him for nothing next year.
And the keeper – while happy at The Hawthorns – has made it clear he’s enjoyed playing in the Premier League and wants to be there long term.
West Ham and Watford are both in the market for a new glovesman before the 2021/22 season gets underway.
The Hammers want competition for 36-year-old Lukasz Fabianski.
The Hornets, meanwhile, need to clarify their goalkeeping position ahead of their return to the Premier League.
Former Albion shot-stopper Ben Foster started last season as their number one.
But he broke his finger in January and then wasn’t able to get back into the side due to the form of Daniel Bachmann.
And with Foster unlikely to want to stay and play second-fiddle again next season, Johnston has been earmarked for a move to Vicarage Road to compete with Bachmann.
Now West Ham and Watford are understood to be readying bids for Johnstone. And while those bids will be below Albion’s £20million asking price – they will kick-start negotiations.
Last summer Sheffield United paid £18.5million to sign Aaron Ramsdale from Bournemouth – a goalkeeper who missed out on a place in the England squad to Johnstone.
But he had recently signed a long-term deal with the Cherries.
And it’s understood Johnstone’s suitors believe £20million is too much to pay for a keeper who is out of contract in a year.