Hospital patients, staff and visitors will be required to wear face masks again, as cases of Covid-19 have risen "significantly".
The rule will apply at all of Cornwall's hospitals, minor injury units and treatment centres from Monday, 4 July, health bosses said.
Two new sub-variants were blamed for the rise in cases locally and nationally, while services were already "under sustained pressure".
The measure will remain under review.
The Royal Cornwall Hospitals NHS Trust (RCHT) said there had been steady increases in the number of healthcare staff testing positive for Covid
-19, adding to additional strain on resources.
Kim O'Keeffe, dual chief nurse for the trust and for Cornwall Partnership NHS Foundation Trust (CFT) said: "Recently, we've seen a rise in cases again and it's crucial we use the tools at our disposal to help protect not only our patients, but visitors and our colleagues too."
"Making mask wearing a requirement once again for staff is a decision we've taken at a time when we were already under sustained system pressure.
She asked that people "be kind to staff, who've been working incredibly hard to protect patients throughout the pandemic."
CFT and RCHT's current policy on visiting arrangements remains unchanged.
Staff, patients, and visitors are still asked to continue to wash hands frequently with soap and water and use the alcohol gel provided, in addition to wearing facemasks.