The Scottish left-back signed a new four-year contract with the Rams yesterday and was quick to credit McClaren for his performances last season.
The former England boss took over at the iPro Stadium last September, guiding the Rams to third place in the table before they lost in the playoff final to Queens Park Rangers.
"Since Steve McClaren came in I think I have improved as a player as well so I'm thankful for that," Forsyth told the club website.
"Hopefully I can continue to develop – I know there's room for improvement – and I certainly think the defensive side of my game came on under him."
Forsyth joined Derby from Watford last summer for £150,000.