Michael Hector has committed his future to Reading after signing a new three-year contract with the Championship club.
Watford have confirmed the signing of Austria defender Sebastian Prodl from Werder Bremen.
The 21-year-old played 20 times for Danny Lennon's men during a loan spell with the Buddies last season.
Despite offers from several Championship clubs in England, Newton has opted to work under Lennon and his assistant Tommy Craig again.
Newton said: "I had offers from a few clubs in the Championship but I decided to come back to St Mirren, partly because they couldn't guarantee me football.
"At this stage in my career I really need to be playing games, so I think St Mirren are the right club to come back to.
"From the gaffer Danny to his assistant Tommy Craig and the other backroom staff and the lads, they are a great bunch. I really missed that dressing room when I went back to Newcastle in the summer.
"St Mirren gave me my first shot at football and that meant a lot to me, so coming back is best for both parties."