The scheme will also require visitors to the UNESCO World Heritage Site to buy 'named' tickets, which, in the summer, will be divided into morning and afternoon slots.
Pompeii director Gabriel Zuchtriegel told AFP the measure was designed to better manage crowd numbers, leading to a more "sustainable" growth.
"The idea is not to close Pompeii but to expand Pompeii and better manage the flow," he said.
Last year over four million people visited Pompeii – an ancient Roman city buried by ash and rocks when Mount Vesuvius erupted nearly 2,000 years ago.
The figure is expected to be even higher this year.
Some 3.84 million people visited between January and October, including a peak of more than 36,000 on one Sunday when entry was free as part of Italy's Domeniche al Museo scheme.
Zuchtriegel said the numbers caused concerns over visitor and staff safety and the conservation of the site, where many of the buildings – and even some bodies of the estimated 3,000 victims – were perfectly preserved following the eruption.
"We want to guarantee all visitors a high-quality experience. It must never be mass tourism," he said.
Limiting ticket numbers during peak season raised concerns about Pompeii's budget, but Zuchtriegel said the plan was set to have "no negative consequences on takings".
The park hopes to tempt visitors to other nearby sites, with the idea that if only afternoon tickets are available for Pompeii, a tourist might decide to spend the morning elsewhere.
Zuchtriegel said efforts were also underway to open alleyways and streets in Pompeii that are currently closed, which would provide more space for visitors and allow organisers to raise the 20,000 daily limit.
"It is not a measure against growth. We are focusing on sustainable growth, lasting over time," he insisted.
Between April 1st and October 31st, a maximum of 15,000 visitors will be allowed in from 9am to 12pm, and 5,000 from 12pm to 5.30pm.
Many Italian tourist hubs have taken steps to tackle overcrowding, from Venice – which introduced a five-euro entry fee for day trippers during peak periods – to the Cinque Terre, where walking the celebrated Via dell'Amore or 'Path of Love' now requires a ticket.
Comments