Arnold has progressed through the youth ranks with the Royals and signed a new one-year contract extension earlier on Friday.
The 19-year-old is yet to make a first-team appearance for the Berkshire outfit but was an ever-present member of their under-21 side last term.
Wanderers boss Gareth Ainsworth told the club's official website: "I'm excited to be working with him having watched him play a number of times for Reading under-21s last season.
"I feel he's got something. He can play anywhere on the right, is energetic and has pace and strength.
"Hopefully he can follow in the footsteps of the successful loan signings we had last year."
Arnold becomes Ainsworth's second summer signing after goalkeeper Charlie Horlock.