The former Liverpool trainee, 25, has penned a two-and-a-half-year deal and goes straight into the squad for Saturday's home match with Burnley.
The Iron also has the option of extending his deal by a further year and Duffy said: "I'm really over the moon, I can't believe it's happening.
"I just want to get out there and meet all the lads and play football. I can't wait to test myself in the Championship.
"I've been in League Two for a year and a half and its obvious that this is going to be a massive step up."
Source: PA
Source: PA