From Monday, masks will again be required outdoors as well as indoors on the island and restaurant diners will be limited to groups of four following an increase in cases of Covid-19.
"I have just signed a new decree which makes Sicily a yellow zone," said Health Minister Roberto Speranza.
"It is confirmation that the virus is not yet defeated," he said, emphasising the importance of the vaccine campaign and personal behaviour such as social distancing.
Sicily will be classed as "yellow", the second lowest level in a four-level classification system based on rates of Covid-19 infections and hospital admissions.
For two months, all Italy's regions have been classed as the lowest risk "white", but the spread of the highly contagious Delta variant has sparked concern.
Since early August, Italy has required proof of vaccination, recent recovery from coronavirus or a negative test for people wanting to dine indoors or enter museums and sports events.
The so-called Green Pass will also become compulsory for teachers and on trains and planes from September 1.
However, further measures were considered necessary in Sicily, where vaccination rates are lower than the national average and where crowds have flocked for the summer season.
Italy was the first European nation to be hit by coronavirus and has suffered badly, recording more than 129,000 deaths so far, the highest number in the European Union.
But its vaccination campaign has been proceeding rapidly, with almost 69 percent of the population over the age of 12 currently fully vaccinated.
