Root, 25, made his Notts debut in 2016 and his breakthrough came this season when he made 107 from 93 balls against Warwickshire in the One-Day Cup.
Bowler Luke Fletcher has also signed a new two-year deal at Trent Bridge.
Fletcher, 28, was struck on the head while bowling at Edgbaston in July - an injury that ended his season.
Before the incident, he took 36 wickets at 22.44 to help Notts move clear at the top of County Championship Division Two.
"It's a well-deserved contract for Luke because of what he has done in the last 12 months," said Notts head coach Peter Moores.
"He's gone away and looked quite hard at what he can do to get himself into the team.
"We have seen the results of that in the way he bowled in the first part of the County Championship."
Root came through the Yorkshire Academy before being released in 2011 and joining the MCC Young Cricketers.
He is now a regular in white-ball cricket for Notts and Moores wants him to push for a place in the four-day team.
"He's been an effective player for us, particularly in one-day cricket," Moores continued.
"We're looking for him to now push on in both white and red-ball cricket after working very hard for this opportunity."
Billy Root, the younger brother of England captain Joe, has signed his first professional contract, agreeing a two-year deal with Nottinghamshire.