Jim Goodwin believes the capture of Eamonn Brophy is a "real coup" for St Mirren.
The Paisley club have announced the signing of the Kilmarnock forward on a pre-contract.
The 24-year-old will join Killie's Premiership rivals on a three-year deal in the summer when his deal expires.
Brophy, capped for Scotland, began his career at Hamilton before moving to Rugby Park in 2017.
Manager Goodwin told St Mirren's official website: "We're absolutely delighted.
"It's a really exciting signing for us considering the teams we were competing with and how highly regarded Eamonn is on the Scottish stage. I think it's a real coup for us.
"He's at a brilliant age and coming into the best years of his career.
"It shows the ambition of the club and I'm just delighted we were able to pull it off."
St Mirren chief executive Tony Fitzpatrick added: "We hope supporters see this signing as a reflection of our ambitions as a football club going forward and another important step in our journey towards fan ownership.
"This deal would not have been possible without the continued support from SMiSA, Kibble, and, most importantly, our loyal fans."
Meanwhile, Junior Morias has joined Boreham Wood on loan until the end of the season.
The 25-year-old forward joined the Buddies from Northampton on a two-year-deal in August 2019.
Morais scored twice in 47 appearances for St Mirren.
The move is subject to international clearance and a statement on St Mirren's official website concluded: "Everyone at St Mirren would like to wish Junior all the best during his loan spell."
