A long-time area resident, Davis previously ran for the Liberals in the October 2011 election.
The byelection, to fill the seat vacated by Progressive Conservative Elizabeth Witmer, is set for Sept. 6.
Witmer resigned after Premier Dalton McGuinty appointed her chair of the Workplace Safety and Insurance Board.
The riding of Vaughan outside Toronto will also hold a byelection on the same day.
That seat became vacant after veteran Liberal Greg Sorbara resigned last week.