The 25-year-old played in every game for the Rams last season as they lost in the Championship playoff final.
Their hopes of gaining promotion back to the Premier League during the 2014-2015 campaign have been boosted by securing the futures of several key personnel, with Forsyth following midfielder Jeff Hendrick in committing himself to the Pride Park outfit for four more years.
Head coach Steve McClaren told the club's official website: "We're very pleased that Craig Forsyth sees his long-term future with Derby County.
"Off the back of Jeff Hendrick penning a new deal on Thursday we're pleased that another key member of last season's team has committed their future to the club.
"Craig has worked hard at his game and that was evident in his performances, especially in the second half of last season, as he played a key role at both ends of the pitch."
Forsyth joined Derby from Watford last summer, for a fee of £150,000.