The 63-year-old has moved to Old Trafford from his role as head of sports and development at Lokomotiv Moscow, and will stay on at the club for a further two years in a consultancy role once his time in the dugout is over.
The post is Rangnick's first managerial position since leaving RB Leipzig in 2019, taking over from Michael Carrick, who won one and drew one of his two games in charge following the sacking of Ole Gunnar Solskjaer.
Here, Sports Mole looks at the key questions surrounding Rangnick's appointment.
When is Ralf Rangnick's first game in charge of Manchester United?
As with most deals when they are announced, there are one or two formalities still to take place before Ralf Rangnick can enter the Manchester United dugout for the first time as interim boss.
In this case, the German must first complete his work visa requirements, and he faces a race against time to do that before Manchester United's meeting with Arsenal on Thursday night.
The hope is that everything will be in order for Rangnick to be in the dugout for that match at Old Trafford, although Carrick will remain at the helm until that issue is sorted.
Should the paperwork drag on then Rangnick may have to wait until Sunday, November 5 - when Manchester United host Crystal Palace - for his first game in charge.
How long is Ralf Rangnick in charge of Manchester United for?
Initially, Rangnick has been appointed on an interim basis until the end of this season, before moving upstairs into a consultancy role.
No deal has yet been agreed as to a permanent successor from the end of the current campaign, although Mauricio Pochettino has been heavily linked with the position, while a plethora of other names have also been included on Man United's shortlist.
There have been reports that Rangnick himself could be in line for the job on a permanent basis if he impresses in the role, although he is more likely to help determine who will succeed him in the dugout.
What has he said about the job?
After his announcement, Rangnick told the club's official website:
"I am excited to be joining Manchester United and focused on making this a successful season for the club. The squad is full of talent and has a great balance of youth and experience.
"All my efforts for the next six months will be on helping these players fulfil their potential, both individually and, most importantly, as a team. Beyond that, I look forward to supporting the club's longer-term goals on a consultancy basis."
What position does he find Manchester United in?
Manchester United have steadied the ship somewhat since Solskjaer departure, but there is still plenty of work to be done by Rangnick at Old Trafford.
The 20-time English champions find themselves eighth in the Premier League table, already 12 points off leaders Chelsea, and having won just one of their last eight top-flight outings.
It is not all bad news, though; Man United are through to the knockout stages of the Champions League with a game to spare, while the gap to the top four is a more manageable five points.
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