Matt Taylor has left his role as Scotland assistant to join Australia, with former Ospreys head coach Steve Tandy named as his replacement.
Taylor had worked beside head coach Gregor Townsend since taking up a dual role as defence coach with Glasgow Warriors and Scotland in 2012.
Tandy has been Super Rugby side Waratahs' defence coach.
Former France prop Pieter de Villiers has also joined the Scotland set-up as scrum coach.
Australia-born Taylor will again work under current Glasgow head coach Dave Rennie, who will become Wallabies boss when the New Zealander leaves the Pro14 club at the end of the season, and alongside attack coach Scott Wisemantel.
Rugby Australia director of rugby Scott Johnson, the former Scotland head coach, welcomed the 47-year-old "home".
"He's a quality coach who has the experience, skillset and personality that we believe will fit really well in the coaching team we have assembled," he said.
Taylor said the highlights of his time in Scotland would be Scotland's Calcutta Cup win over England in 2018 and winning the Pro12 title in 2015 with Glasgow.
"I'm going to miss Flower of Scotland, the roar of BT Murrayfield, the passion of Scotstoun and - most of all - the celebrations in the changing room after a game with players and staff alike, but the time is right to take on a new challenge," he said.
"I have supported the Wallabies since childhood, so to be part of this group moving towards the Rugby World Cup in France in 2023 is an exciting prospect."
Townsend recalled how he and Taylor, who also played for Edinburgh and Scotland A, became close while with Border Reivers.
"As teammates back at the Borders, we talked a lot about the possibility of going into coaching, then we used to share notes and learnings as we began our coaching careers on opposite sides of the world," he said.
"Matt is passionate about coaching - helping players and the team improve - and also continually striving to be a better coach. He can be really proud of a number of achievements he helped create over an eight-year period in Scottish rugby."
'Massive opportunity' for Tandy
Taylor's replacement, Tandy, was Ospreys head coach for six seasons and led them to the Pro12 title in 2012.
"He was a successful head coach in the Pro14 and added another impressive dimension to his coaching as an assistant coach in Super Rugby, which is a rare move for a northern hemisphere coach," Townsend said.
The 39-year-old Welshman sees his move to Scotland as "a massive opportunity".
"Being part of Super Rugby for the past two seasons has enabled me to gain invaluable experience in different styles of rugby," he said.
"Playing against teams from South Africa, New Zealand, Japan, Argentina and Australia has been an amazing learning opportunity for me and will aide my transition into the international game."
South Africa-born De Villiers has been scrum coach with the Springboks and Stade Francais but welcomed the opportunity to be "back in the midst" of Six Nations competition.
Townsend might see this as a coup - analysis
Senior sports reporter Andy Burke on BBC Radio Scotland
The thing that has caught a lot of us by surprise is that Taylor is leaving his job immediately. We thought that he would see out the Six Nations - given the Six Nations is not far away at the start of February. That doesn't seem like a long time to bring new people into the camp and get their new ideas across.
Matt Taylor has given a lot to Scottish Rugby. He was part of the coaching team that led Glasgow to the PRO12 title in 2015 and the fact that four successive Scotland head coaches have wanted him by their side tells its own story, but it might be a case of it is a move that suits both parties at this stage - the Six Nations, the World Cup, none of it has gone to plan.
The defence has not been working for Scotland, the same mistakes kept on happening and there is maybe a sense that the players being exposed to new voices and new ideas might help get the Six Nations campaign off to a better footing.
On the face of it, Steve Tandy looks a very good appointment. Gregor Townsend will think of it as being a bit of a coup getting him in - of course, the proof will be in the pudding come that Six Nations opener against Ireland whether we see significant defensive improvements.