Sammon has penned a four-year deal with the Rams.
Derby boss Nigel Clough told the club's official website: "When we first drew up a list of potential options with regards to bringing in a new striker it's fair to say Conor was top of the list, our number one target actually, so we're delighted to announce his arrival.
"Having tracked him closely at Kilmarnock and, as everyone knows, being 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."