CBC.ca will live stream the news conference starting at 9 a.m. ET. Click on the image that appears in this story to watch.
The NDP's share of the popular vote increased slightly, but the party failed to add to its 21-seat total.
Kathleen Wynne's Liberals finished with 58 seats, enough to return to power with a majority and shut the NDP out of future budget discussions.
After this morning's news conference Horwath is scheduled to meet with her caucus, where she's likely to face questions about her decision to not support the minority Liberals' budget, a move that triggered the election.
NDP officials denied rumours circulating Tuesday afternoon that Horwath would announce her resignation today.