Villa boss Steve Bruve has raided his former club to sign the 29-year-old, who he managed at Sunderland and Hull.
Elmohamady, who's signed a three-year deal at Villa Park, will now link up with the squad in Germany.
"Aston Villa is a great club and I am thrilled to join, it's one of the very biggest clubs in England and its history and tradition is there for all to see," said the Egyptian.
"It's a great time to join too as we look for promotion to the Premier League."
I will give absolutely everything to help us achieve that goal."
Elmohamady is Villa's third signing of the summer following the captures of John Terry and Sam Johnstone.