Mourani is urging sovereigntists to join the Bloc in order to help it rebuild after its crushing electoral defeat.
She was one of only four Bloc candidates to emerge victorious in last May's general election.
Mourani joins fellow MP Jean-Francois Fortin in the race to succeed Gilles Duceppe.
The new leader will be chosen in December.
The 42-year-old Mourani has been a member of Parliament since 2006.
She has been the party's longtime critic on women's status and public security, and has now added various other portfolios.
Mourani is a former probation officer who has written extensively about street gangs.
She was born in Ivory Coast and is of Lebanese descent.