The lockdown that will affect over 200,000 people in the region.
This is the first such measure to be introduced since Spain ended its State of Alarm allowing the return of free movement around the country on June 21.
Regional leader Quim Torra explained that the decision has been made after a "considerable increase" in the number of coronavirus cases in the Segria region, which covers Lleida, its capital, and the surrounding area of 38 municipalities.
Almost 350 COVID-19 cases have been reported in the Segria region over the past week, many of them among temporary fruit pickers, and a temporary field hospital has been set up in Lleida.
The lockdown will last for an initial period of 15 days. Miquel Buch, head of the Catalan interior department, has confirmed an end to free movement "from 12:00 hours on July 4, 2020 till a new resolution is adopted."
Around 200 members of the Catalan regional police force (Mossos d'Esquadra) will be deployed at various points to enforce the lockdown.
Catalonia is the second region in Spain hardest hit by the coronavirus.