The 22-year-old from Sweden was the 29th overall selection in the 2018 draft. Sandin had 16 points in 51 games with Toronto last season. He's played in 88 career regular-season games, with six goals and 22 assists, and has one goal in five playoff games.
"Got a great set of tools," fellow defenseman Jake Muzzin said. "With experience, I think they're only going to get better."
The signing comes as the Leafs' blueliners been hit hard by injuries. Muzzin has been dealing with a back issue, and Timothy Liljegren recently had surgery for a hernia.
Toronto then lost Jamie Benn (groin) and Carl Dahlstrom (shoulder) in Wednesday's 3-0 preseason victory over the Montreal Canadiens, pressing forwards Calle Jarnkrok and Alexander Kerfoot into defensive roles for two periods.