Messi joined the Blaugrana aged 13 and made his senior debut three years later as he embarked on what has so far been an incredible playing career.
As reported by Spanish radio show El Larguero, the new deal won't be signed until after 30 June, for accounting reasons, with the related costs being booked next season.
He has since scored more than 500 goals for the club.
The Catalan club's spokesman Joesp Vives confirmed in May that the club are very close to agreeing a deal with the five-time Ballon d'Or victor. Barca finished second in LaLiga in the 2016/17 campaign, surrendering their crown to arch rivals Real Madrid, and were knocked out of the Champions League at the quarter-final stage by Juventus.
The 23-year-old has established himself as one of La Liga's most consistent centre-backs, flourishing under Valverde's tutelage during the Barcelona boss' time at the club.
"I spoke to him once and always said to push the forwards", Guardiola said. I would love to finish my career at Barcelona, ' he told Tencent Sports.
Messi, now away on worldwide duty with Argentina, added: "We would have liked to have achieved more".
The current Manchester City manager won a trio of LaLiga titles and lifted the Champions League twice with a Messi-inspired Barca team between 2008 and 2012.