There has been much speculation about Dexter's future after the NDP were knocked from power to third-party status in last month's election, winning seven of the legislature's 51 seats.
Dexter also lost his own Halifax-area seat of Cole Harbour-Portland Valley.
His announcement will be made at NDP politician Maureen MacDonald's constituency office, who has taken on the role of leader in the house.
Party president David Wallbridge would not comment on the nature of the announcement.
Dexter won the party leadership in 2001.