Pompey Academy’s nine-match pre-season unbeaten run came to an abrupt end in the Midlands.
The Blues twice took the lead but still succumbed to a heavy defeat.
Ryan Williams had swept Paul Smalley’s side into an early lead but Derby levelled midway through the first half.
Pompey again went ahead when Lewis Stockford (pictured) fired home but the hosts struck twice to hold a narrow advantage at the interval.
And Derby took advantage of some goalkeeping slips to ensure a result that was more emphatic than the Blues’ performance deserved.