The right-back, who spent the second half of last season on loan at Hibernian from Dundee United, has agreed a deal until January.
Watson, 25, is in line to make his debut at Cappielow tonight. With Jason Naismith ruled out with injury for six months, St Mirren manager Ian Murray said he was delighted to secure the services of Watson who can also play in central defence.
“Keith was someone we looked at a couple of weeks ago in terms of someone who could play several positions and once we knew we needed a right-back in he was our first name,” Murray told the St Mirren website. “To get it done now in time for tomorrow is great.
“We’ve signed him, he’s ready to go, he’s fit and that’s the main thing so he certainly comes into our thoughts for tomorrow night.
“We needed someone to come in and play in that area and play in that position so to get someone of Keith’s calibre, pedigree and experience is great.”
Livingston-born Watson came through the youth ranks at Tannadice before going on to make over 120 top-flight league appearances for United. He spent the second half of last season on loan at Hibs and looked like he might make the move to Easter Road a permanent one.
However, he was injured during pre-season and has now moved to Championship rivals St Mirren.
He said: “It’s been a long summer and I’m absolutely delighted to come here, do well for St Mirren and hopefully get as much games as I can.
“I’d done quite a bit of training with Hibs over the summer and got quite a lot of running there and I was also in Aberdeen for a couple of weeks and trained with them so I think my fitness will be fine.”
St Mirren are also in talks with free agent Andy Webster about a possible move to Paisley.