Longmuir, who took the reins in September 2019, has added another two years to a deal that was due to expire at the end of 2022.
The re-signing comes less than a month after the Dockers outlined their vision to claim maiden AFL and AFLW premierships by 2025 as part of an ambitious strategic plan.
Under Longmuir, Fremantle finished 12th in 2020 and 11th in 2021.
"This contract extension doesn't really change my approach, it's just nice to know that board and management are happy with the work I'm doing so far," 40-year-old Longmuir said in a statement.
"This isn't my club, it's everyone's club and I've got to do the right thing by Fremantle and everyone who supports us.
"As senior coach, that is my aim and I'll continue to do that.
"Our responsibility is to make sure we set up an environment where all of our players can be their best selves and reach their potential.
"If we can do that, things do look very exciting. But there's a lot of work that has to go into achieving our goals. We have to keep doing the work.
"I'm pleased with where we are at but not satisfied at all."
Longmuir played 139 games for the Dockers between 1999 and 2007, before moving into coaching, first at West Coast, then at Collingwood.
"Justin has a great balance of knowing the game incredibly well and being a technically strong coach while also having the ability to build genuine relationships and a high level of rapport with the players and football staff," chief executive Simon Garlick said.
"While we believe Justin has great capability as a coach. We also feel he is a great fit for our club from a culture and values perspective
"We are very pleased with the work Justin has done thus far and while we all know there is much to do, we feel confident in the direction the football program is heading."
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