The star hooker would have been free to negotiate with NRL rivals from November 1 but said it "was not a hard decision" to stay put at least until he is 29.
Grant had an uncharacteristically quiet start to the 2024 season, before finishing strongly to claim Dally M Hooker of the Year en route to a maiden grand final appearance.
The Queensland State of Origin representative scored the Storm's only try in a 14-6 loss to Penrith earlier this month.
Grant assumed the Melbourne captaincy from Christian Welch this season and is Australia's vice-captain behind Isaah Yeo for the ongoing Pacific Championships.
Our Captain stays on. 2026 💜— Melbourne Storm (@storm) October 23, 2024
"It wasn't a hard decision to make given the club has put trust in me with the captaincy this season," Grant said on accepting his player option.
"We have a lot of natural leaders in the squad too which helps both on and off the field. The potential and depth of our roster really excites me."
Grant has played 86 of his 101 career games for the Storm, having first garnered league-wide attention playing for Wests Tigers as part of a historic loan deal in 2020.
Grant won the Dally M Rookie of the Year that season before returning to assume starting duties from Cameron Smith after the champion hooker's retirement.
Elsewhere, second-rower Jackson Ford has signed a two-year extension to remain with the Warriors until the end of the 2027 campaign.
Ford enjoyed a breakout season on arrival from St George Illawarra in 2023 and follows Dallin Watene-Zelezniak, Chanel Harris-Tavita and Ali Leiataua in re-signing with the Auckland side.
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