The 25-year-old Irishman, who cost the Rams an undisclosed fee, has signed a four-year contract at Pride Park.
Discussing his latest recruit, Derby boss Nigel Clough revealed that Sammon was his "number one target" for the summer transfer window.
"When we first drew up a list of potential options with regards to bringing in a new striker it's fair to Conor was top of the list, our number one target actually, so we're delighted to announce his arrival," Clough told the club's official website.
"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."
The striker is expected to make his debut against Bolton Wanderers tomorrow night.