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."

1 July 2015 Last updated at 16:19 BST
Mark Sampson's squad are only the third England team to reach a World Cup semi-final, after the 1990 men's side and the 1966 World Cup winners.
The winner of the match will face USA in Sunday's final
Sue Smith, a former England player who is commentating on tonight's semi-final, told Newsround that the World Cup had been a 'game changer' for women's football.

Greybull Capital, a UK-based investment firm, is to plough up to £400m into the plant - but workers had to accept a pay cut and smaller pension contributions.
In an earlier ballot, Community, Unite and GMB members accepted temporary changes to their terms and conditions.
Officials said it was a positive step in "securing a sustainable future" for the plant.
More on this and other local stories in Lincolnshire
Unite's National Officer, Harish Patel, said: "This will have been a difficult decision to take for many, but by agreeing to make these short-term sacrifices, members have secured a future for steelmaking in Scunthorpe and the long product division's other sites.
"Government ministers need to make sure that the sacrifices are not being made in vain by taking decisive action to support the steel industry and allowing steelworkers to compete on a level playing field with their global competitors."
Steve McCool, from the union Community, echoed calls for Government action.
"He said: "The steel made at Scunthorpe and across the north of England is some of the best in the world and is absolutely vital to the infrastructure and construction industries."
"When Britain builds, we must always build using British steel," he said.
The Government is yet to respond to a request to comment on what the union leaders have said.
Ahead of the vote, steelworker Charlotte Upton said the proposed deal meant "job security for me so I can stay where my family is, near my home".
"It means I can continue to be a steelworker, I love my job."
The proposed temporary changes to terms and conditions include a one year, 3% reduction in pay, and a one year, 3% reduction (from 6%) in both employer's and employee's pension contributions.
The Tata signs will be also removed and replaced with ones saying British Steel.
The Scunthorpe steelworks is part of Tata Steel's long products division, which was put up for sale in 2014.
Billy Root, the younger brother of England captain Joe, has signed his first professional contract, agreeing a two-year deal with Nottinghamshire.
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England women's football team will take on Japan on Thursday in the semi-finals of the World Cup in Canada.
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Workers have voted to accept a deal which will safeguard about 4,400 jobs at Tata's steelworks at Scunthorpe.