St Johnstone have secured the signing of St Mirren defender Keith Watson on a pre-contract.The 26-year-old will join the Perth club in the summer on a deal that will keep him at McDiarmid Park until summer 2018. Manager Tommy Wright said in April the club had opened talks with Watson when he also reiterated his ongoing interest in Falkirk goalscorer Blair Alston and Dundee United’s Ryan Dow.Watson’s St Mirren contract is due to expire at the end of the month. He joined the Paisley side at the start of the season and featured 29 times, scoring five goals. The right-back made over 100 appearances for Dundee United, where he came through the ranks, and won the Scottish Cup with the Tannadice side in 2010. Watson leaves Alex Rae’s St Mirren side side following their sixth place finish in the Championship which drew to a close at the weekend.