The left-sided full-back has joined from Liverpool, signing a deal until 2024.
It comes a day after Ali Crawford made his loan move from Bolton Wanderers permanent, while Reece Devine and Lars Dendoncker had their loans cancelled.
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Hide AdDavidson is looking to reshape his squad with the team bottom of the table after a disastrous first half of the season.


Gallacher moves north having been at Liverpool since he moved to the club from Falkirk for a six-figure fee in 2018.
He made one first-team appearance for the Anfield club and enjoyed a short-loan spell with Toronto FC in MLS.
"I'm really excited about my move to St Johnstone and can't wait to get started," Gallacher told the club's website.
"I had a great time at Liverpool and I'd like to thank everyone there for looking after me and helping me to improve as a footballer and teach me so many good habits.
"I feel very lucky to have been part of a massive Football Club with so many wonderful professionals and people. It was an incredible education.
"But I felt the time was right to look for first-team football and an exciting opportunity presented itself at St Johnstone.
"Having been out on loan at Toronto, I enjoyed my time there and it made me want more and more first-team football.
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Hide Ad"It's now up to me to show everyone at St Johnstone that I'm good enough to play regular first-team football and that's what I plan to do.
"My preference is to play on the left-hand side as I feel I have good energy to get up and down and can deliver crosses into the box. I also enjoy the defensive side of the game.
"My immediate aim is to work hard and integrate with the group. Team spirit is so important and I know it's a really good squad at St Johnstone.
"There will be a feeling of positivity going into the second half of the season. We then need to take that into games and we will.
"It's vital to win games of football and I am confident we will do that to move away from the bottom end of the table."