Defender Dean was released by Southampton over the weekend and midfielder Forshaw, also 20, has joined from Everton.
Both players had successful loan spells at Griffin Park last season and have now signed two-year contracts.
Manager Uwe Rosler told the club website: "We have worked very hard in securing Harlee and Adam. They have proved during their time here that they are very talented young players and I am delighted to have them both on board.
"Both of them are players with a lot of potential and they are both at the start of their careers and will get better.
"They each showed consistency while on loan with us and now they have the opportunity for regular first-team football."
