Also, forward Tim Connolly cleared waivers and has been assigned to the Toronto Marlies.
The Leafs will begin the season with 13 forwards, eight defencemen and two goaltenders. Defenceman Jake Gardiner is not included as he is currently an injured non-roster player.
Rielly was selected with the fifth overall pick in the 2012 NHL Draft. The Vancouver, B.C. native had three goals and 18 points in 18 games with Moose Jaw in the WHL last season.
The 18-year-old Rielly also played for Team Canada in this year's World Junior Hockey Championship, posting a goal and two assists in six contests.
Frattin, 25, had eight goals and seven assists in 56 games with the Leafs last season.
He has spent his entire career in the Leafs' system, after being drafted in the fourth round of the 2007 NHL Draft.