The Rams lost out to Wigan in the race for Sammon's signature 18 months ago, with the Irishman opting for a switch to the Premier League from SPL side Kilmarnock.
But Derby manager Nigel Clough has finally got his man, signing Sammon on a contract until June 2016.
"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," said Clough.
"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."
Sammon was at Pride Park on Saturday to watch Derby begin their npower Championship campaign with a 2-2 draw against newly-promoted Sheffield Wednesday.
The 25-year-old, Clough's seventh summer signing, will go straight into the Rams' squad to face Bolton at the Reebok Stadium on Tuesday evening.
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