The 63 year old German will takes over from Solsjær who was sacked some days back, subjected to a work visa been issued.
Following this period, Ralf and the club have agreed that he will continue in a consultancy role for a further two years.
John Murtough, Manchester United football director, said: “Ralf is one of the most respected coaches and innovators in European football. He was our number one candidate for interim manager, reflecting the invaluable leadership and technical skills he will bring from almost four decades of experience in management and coaching.
“Everyone at the club is looking forward to working with him during the season ahead, and then for a further two years in his advisory.
Ralf Rangnick said: “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.”