Horgan announced on Twitter that the Opposition party should allow the younger generation to shape the future.
The Vancouver Island politician who is also house leader plans to remain as MLA.
Horgan, along with NDP MLA Mike Farnworth, was considered a possible front-runner to replace Adrian Dix, who announced in September that he will step down when the party chooses its new leader sometime in 2014.
Before that, a party-sanctioned review team will produce a report examining why the New Democrats lost the provincial election in May after being widely expected to win.
Horgan ran for leadership of the NDP in 2011, when Dix was chosen to replace Carole James.
Note to readers: This is a corrected story. An earlier version said the leadership convention would be in November.
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