The 26-year-old, on the Terriers' books since 2012, made 16 appearances in last season's promotion-winning campaign.
However, he did not feature in the Championship playoffs or in last weekend's opening Premier League game at former club Crystal Palace.
Burton manager Nigel Clough told the club website: "We are delighted to have got in such a good player and one that is coming from a club who are now in the Premier League.
"He is a player who has impressed us over a number of seasons and who has always played well against us. He can play wide or through the middle and has pace. He will provide us with a much-needed attacking threat."
Scannell, meanwhile, said: "I'm just looking forward to getting back out on the pitch and playing. I'm really excited.
"I like to get forward, I'm an attacking player and I like to use my strength and get up the pitch. Hopefully throughout the season I can show what I can do for Burton Albion."
Scannell is the seventh signing of the summer for the Brewers, who are bottom of the Championship after losing their first three games.