Horgan will be running against veteran MLA Mike Farnworth. Both fought for the job in 2011, but eventually lost to current leader Adrian Dix.
After Dix announced he was stepping down last September, Horgan was considered a likely candidate for the leadership, but instead he announced he was stepping aside to allow room for new younger candidates.
But since that time none have come forward, and Horgan could have a solid shot at winning. The only other candidate in the race is Farnworth, who announced he was running earlier this month.
Horgan is considered to be more pro-economic development than current leader Adrian Dix, and has already talked about re-examining Dix's decision to oppose the expansion of the Kinder Morgan oil pipeline.
Horgan was first elected in 2005, but worked behind the scenes when the NDP was in power in the 1990's.
The party will pick its new leader in September.