Ottawa-area MPP Lisa MacLeod launched her campaign on Sunday.
MacLeod already signalled earlier this fall that she would be a candidate to succeed Tim Hudak, who resigned after his party's crushing defeat in the June provincial election.
MacLeod made the announcement to supporters in Ottawa and said in a statement that she hopes to restore her party and make it a viable contender in the next election.
Caucus mates Christine Elliott, Vic Fedeli, Monte McNaughton and Conservative MP Patrick Brown have already announced they're running for the leadership.
The vote will be held May 3 and 7 and the results are to be released May 9.
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