The 22-year-old has agreed a two year deal. He started just four times during his time at Elland Road and had loan spells at Sunderland, Doncaster Rovers and Barnsley last season.
Manager David Hodgson said: “Simon is a young talented striker and has good experience of league football and will be a key member of our squad for the new campaign.
“We’ve been trailing him for sometime and I’m delighted that he’s decided to join Darlington. We are still in talks with our other targets and hope to have some more good news for the fans in the next few weeks.”