The former Cardiff frontman failed to secure a regular starting place under both former Rams boss Billy Davies and current manager Paul Jewell, and only managed two goals all season.
Unsurprisingly he was thrilled to get the chance to make a fresh start.
"Derby was disappointing but that's the way it goes," Earnshaw told the club's website. "I'm just looking at is as a chapter closed and will learn from it.
"Getting to know Colin Calderwood was important. I wanted to know what he was looking for.
"We had a good, long chat and that was an important factor in my decision in the end."
Forest chairman Nigel Doughty added: "When you look at Robert's record in terms of goals per game he's probably the most successful Championship player.
"Allied with the strikeforce we have, I think we will be a potent goalscoring force next season."
Meanwhile, Forest have announced their second pre-season friendly against Premier League opposition.
They will face Everton at The City Ground on Friday July 25th, just six days before they face Roy Keane's Sunderland.
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