The 25-year-old, a long-term target for Rams boss Nigel Clough, has signed a four-year deal at Pride Park and will be available for the Championship outfit's game against Bolton on Tuesday.
Sammon made 32 Premier League appearances for Wigan since joining from Kilmarnock in January 2011, scoring just once.
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"Having tracked him closely at Kilmarnock and, as everyone knows, been so close to bringing him in 18 months ago, we've monitored his progress at Wigan in the Premier League and we know he is capable of finding the back of the net.
"He is going to be a key player for us and when he plays regularly, we know he can score goals. We really like how hard he works too and that means he will fit into our squad comfortably."