Essex 149 for 5 (Pepper 48) beat Sussex 147 (Bopara 50, Cook 4-20) by five wickets
Pepper was the Eagles' top scorer in 2022 with 439 runs and has belatedly started 2023, after a finger injury, with 63 against Somerset and a stylish 48 from 26 balls at Chelmsford.
Matt Critchley and Daniel Sams completed the win with 31 balls to spare as Sussex were condemned to a fourth straight defeat, as Essex chalked up their third victory from five matches.
In the chase, Dan Lawrence was caught at mid-on second ball but Pepper and Rossington got the Eagles off to a flier with 77 runs coming in the powerplay. Henry Crocombe was Rossington's victim with a variety of scoops and pulls to and over the boundary in a 21-run over.
Nathan McAndrew also was sprayed for 21 at the other end in a 10-ball over with Pepper using his crease to create angles to crash four fours.
Rossington flamingoed his fourth six but his stay ended when he turned Tymal Mills around the corner to short leg, before Robin Das and Paul Walter followed quickly.
Pepper popped Bopara back over his head but Shadab Khan pinned him lbw on the reverse sweep. But Critchley and Sams eased to the finish line with a 45-run stand.
Sussex chose to bat and lost Toms Clark, Haines and Alsop in the powerplay but Bopara steadied things with Shadab Khan and Michael Burgess.
Bopara played 67 home T20 matches for Essex, having played from the inaugural 2003 season to 2019, with all but one coming at the Cloud County Ground, Chelmsford. He knows the dimensions of the ground better than most.
On this occasion he targeted the leg side boundary on the Doug Insole Pavilion side, dragging Ben Allison for six from outside the off stump before to slog sweeps off Simon Harmer. He added 35 with Shadab, and after the Pakistani was bowled by Walter's first ball, Burgess helped bring 40 in 24 balls.
Bopara's 46th T20 fifty came in 30 balls but he departed when Shane Snater grabbed some air to catch at long-on.
Sussex would only score another 33 in the last six overs as Critchley had James Coles and McAndrew in the 16th over, Cook returned four-for as he had Fynn Hudson-Prentice and Burgess caught in the same over.
Sams got the wicket his tight bowling deserved when Mills chipped to a diving Harmer at cover as Sussex were bowled out for 147 with three balls unused.