The Republic of Ireland under-21 international has signed a three-year deal and will join the Terriers for an undisclosed fee subject to a medical.
The winger, who can also play up front, made 141 appearances and scored 13 goals for the Eagles after graduating from their youth set-up in 2007.
Scannell, aged 21, was a key figure for Dougie Freedman’s Championship team last season making 41 appearances and scoring four goals. During his time with the Eagles he won the Young Player of the Year award in 2008 and the Championship Apprentice of the Year accolade in the following season.
Grayson said: “We are delighted to welcome Sean to the club, he is a brilliant addition. We have been working hard on a number of targets and hopefully we will be able to get a few more tied up over the next couple of days.
“He is an exciting, young, talented footballer who is very adaptable. He can play on either wing or just off the striker, he offers great options.”
Grayson added: “Despite his young age he brings bags of experience at both Championship and international level and hopefully he will be able to fulfil his potential with us.
“It wasn’t that long ago that a lot of top class clubs were taking a look at him so we are delighted to now have him amongst our ranks.”
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