Derby claim the man is an employee of their Championship promotion rivals, who play each other on Friday evening.
Police were called following reports that the man was “acting suspiciously”, the club added.
Manager Frank Lampard stopped training until the incident had been resolved.
Derby say they are “in discussion” with officials from Leeds about what happened.
Derbyshire Police said the man was found “at the perimeter fence of Moor Farm and was spoken to”.
The force added : “No arrests were made and the man was sent on his way.
“Despite media reports, no damage to the fence was found, and the man was stopped outside the grounds.”
In another tweet, it added, tongue in cheek: “Keeping the team safe to bring home a win against LUFC on 11th!
“Spying is cheating.”
“Derby County Football Club can confirm that officers from the Derbyshire Constabulary were called to the Derby County Training Centre at around 11:20am on Thursday 10th January 2019 following reports of a man acting suspiciously outside the premises,” the club said in a statement on its website.
“It has since been confirmed to Derby County that the individual concerned is an employee of the footballing staff at Leeds United Football Club.
“The club is now in discussion with Leeds United club officials in relation to this incident.
“At this time no further comment will be made.”
Leeds United have not commented on the story.
Derby have not made a complaint to the English Football League, despite reports to the contrary.
“It is, of course, up to Derby County as to how they progress this matter but as of yet we have received no complaint or contact from the club,” an EFL spokesman said.
Derby occupy the final playoff place in sixth, but can close to within three points of automatic promotion with a win at Elland Road.
Leeds have lost their last three games in all competitions.
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