The 27-year-old forward was unveiled to the media on Wednesday after talks with United progressed last week.
Barcelona hold an option to make the move permanent next summer at a reported fee of €30 million (£26m).
Rashford's wages will also will be covered for the duration of the loan and it is thought he has taken a 25 per cent pay cut to join the Spanish giants.
Barcelona has long been a preferred destination for Rashford, who will be embarking on a second straight loan spell.
He spent the second half of last season on loan at Aston Villa after dropping down the pecking order at United under Ruben Amorim.
A move away from Old Trafford seemed a given this summer with his number 10 shirt handed to new signing Matheus Cunha.
However, Barcelona are currently unable to register Rashford to compete in LaLiga, although he is able to train and play in non-competitive games.
LaLiga's financial rules state clubs can only spend as much as they earn, meaning Barça must find other revenue streams or make player sales to fit in their new signings.