The Former Dockers forward replaces Ross Lyon, who was sacked in August despite having a year to run on his contract.
Longmuir was chosen ahead of Gold Coast assistant Dean Solomon and Under 18s coach Peter Sumich.
The 38-year-old has spent 10 years building his coaching apprenticeship, most notably at West Coast (2011-17) and more recently Collingwood (2018-19).
He earned accolades for the way he moulded Collingwood’s defence last year.
Longmuir said he was honoured to be coaching the club where his playing career began in 1999.
And he feels like he has some unfinished business at the Dockers after having to retire at the age of 26 because of a degenerative knee injury.
“It means everything to me to coach Fremantle,” Longmuir said.
“Fremantle gave me my first opportunity to play in the AFL and for that I’m extremely grateful.
“I left the club knowing there was unfinished business here and I’ve returned determined to repay the faith that the club has again shown in me.
“I’m excited to get to work and get to know everyone around the club. It’s obviously changed a lot since I left but I’m genuinely excited to be back.”
The No.2 pick from the 1998 draft, Longmuir played 139 games for Fremantle between 1999 and 2007.
Longmuir’s most famous moment came in 2005, when he took a big pack mark and then kicked truly from 30m after the final siren to help Fremantle beat St Kilda by five points in round 21.
Fremantle president Dale Alcock said Longmuir was the standout coaching candidate.
“Justin stood out for us as someone who has been preparing for this role for the past 10 years,” Alcock said.
“He’s had a successful career at both West Coast and at Collingwood more recently and he knows what’s required to win and what’s required for sustained success.
“With the reinvigoration and enthusiasm Justin will bring to the playing group combined with a fit and available list, we will head in to the 2020 season without any limit on what is possible.”
Longmuir inherits a list boasting plenty of talent but also gaping holes, especially through the midfield.
Fremantle haven’t reached the major round since 2015, and injuries to key players including Stephen Hill, Jesse Hogan, Alex Pearce and Matt Taberner helped kill off a finals charge this year.
The Dockers are expected to lose Brad Hill to St Kilda and Ed Langdon to Melbourne during the upcoming trade week, meaning their need for midfield reinforcements will be even more urgent.
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