In a statement, the PRI said the national convention would be made up of around 19,100 delegates to be chosen during the second week of December. President Enrique Peña Nieto is barred by law from seeking a second six-year term.
Having ruled continuously from 1929, the PRI had become a byword for corruption by the time it was voted out in 2000.
Peña Nieto returned the party to power in 2012, but a slew of corruption scandals, ongoing gang violence, and anaemic growth have sapped the PRI’s credibility over the past five years.
The party faces an uphill struggle to hang on to power, and veteran leftist Andrés Manuel López Obrador, twice a runner-up for the presidency, has led most early polling for 2018.
The run-up to the election takes place in the midst of fraught discussions between Mexico, the United States and Canada over the future of the North American Free Trade Agreement, which underpins much of the region’s commerce.
U.S. President Donald Trump has threatened to walk away from the accord if he cannot rework it in favour of the United States. The three nations have pledged to keep talking through March.