Best was first elected to the legislature in the 2007 election and re-elected as the MPP for Scarborough-Guildwood four years later. She served as a cabinet minister under former premier Dalton McGuinty.
In a statement, Best thanked the people of Scarborough-Guildwood “for their support and friendship,” saying it was an honour to serve them.
The twice-elected MPP did not give a reason for her resignation in the statement.
The governing Liberals have seen five cabinet ministers leave politics since Premier Kathleen Wynne took the reins of the party.
Former finance minister Dwight Duncan and former energy minister Chris Bentley both left the government in February.
McGuinty officially left Queen's Park earlier this month.
Laurel Broten, who served in several cabinet positions, announced last week that she would be stepping down as the MPP for Etobicoke-Lakeshore in July.
Now with the additional departure of Best, the minority Liberals will soon have five fewer seats than they did at the start of the year. That will leave the government with just 48 seats in the legislature, compared to the 36 held by the Progressive Conservatives and 18 by the New Democrats.
Wynne will need to call byelections in Duncan's former Windsor-Tecumseh riding and Bentley's former London West riding by August 15. The premier has said that she would schedule a riding in Ottawa-South, McGuinty’s former riding, at the same time.