He has joined the U's on an initial six-month loan, until 10th January 2015.
Appiah, 23, made 19 appearances for the U's last season, scoring ten goals.
"Obviously I was here before and had a good connection with everyone," Appiah told cambridge-united.co.uk.
"It was great to see the team win at Wembley in the FA Trophy and then be here at the Abbey for the play-off semi-final. Hopefully the fans will see some more goals from me this season and hope as a team we can give them something to cheer about!"
For the big stories and the banter follow us on Twitter: @tribalfootball