Willey has been captain since 2023 and led the team to a finals day appearance last summer, only to lose to Hampshire by six wickets.
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Broad scored an unbeaten 61 off 39 balls in that match and contributed 317 runs throughout the competition, plus three wickets.
“David is a great professional, he pushes the players and sets very high standards within the group,” said coach Darren Lehmann.
“He is a great asset to the club and we look forward to seeing him lead by example again this year.”
He added: “Broady has shown great leadership qualities over the last year and has a brilliant cricket brain.”
Northants, winners of the T20 Blast in 2013 and 2016, begin this summer’s competition with a home match against Glamorgan on May 22.