He sent an email Wednesday to supporters urging them to join him for "an important announcement" on Sunday.
"Friends, I have news to share and I want you to be a part of it," he wrote.
Dewar has made no secret of the fact that he's been assembling a cross-country leadership team and weighing his chances.
The 48-year-old was first elected to Parliament in 2006 and was re-elected in 2008 and 2011. He is currently the NDP's foreign affairs critic.
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So far, only party president Brian Topp and Quebec MP Romeo Saganash have formally declared their intention to succeed Jack Layton, who died last month of cancer.
Other MPs are still weighing their chances, including Thomas Mulcair of Quebec, Peter Julian and Nathan Cullen of British Columbia, Pat Martin and Niki Ashton of Manitoba, Peggy Nash from Ontario and Nova Scotia's Robert Chisholm.
Dewar, the son of late Ottawa mayor and NDP MP Marion Dewar, has deep roots in the party.
He's seen by some party stalwarts as a "unifying alternative" to the two perceived frontrunners, Topp and Mulcair, around whom the contest has polarized in the early days.
However, Dewar is not fluently bilingual and that could prove a handicap in a party that is determined to build on its historic breakthrough in Quebec in last May's election. The NDP took 59 of Quebec's 75 seats and MPs from that province now make up more than half the NDP caucus.
Dewar has called his French "a work in progress."
New Democrats will choose Layton's successor on Mar. 24.