Shivnarine Chanderpaul's 85 not out helped Derbyshire inflict a 63-run defeat on Group B leaders Essex in the Yorkshire Bank 40.
The West Indian hit five fours and two sixes in his 69-ball innings to see Derbyshire up to 321-5 after they were invited to bat at Leek.
The total was a Derbyshire record in the 40-over game and represented the most conceded by Essex in a one-day match.
Opener Wes Durston set the tone for a run-filled day with five sixes in his 46-ball 71 and Alex Hughes contributed 59 not out to an unbroken sixth-wicket stand of 102 with Chanderpaul in the final 10.1 overs of the innings.
In reply, Essex were bowled out for 258 in 33.1 overs.
Mark Pettini top scored with 88, while Mark Footitt (3-40) and Hughes (3-56) each claimed three wickets.
The result moves Derbyshire up to fourth in the Group B standings, two points behind Essex but having played a game less.