Markit/ADACI's services purchasing managers' index (PMI) for Italy climbed to 55.3 in December from 53.4 in November.
A reading above 50 indicates expansion in the sector.
The index was driven by a rise in total activity, new business and job creation.
"With data showing further solid gains in new business and the pace of job creation strengthening, businesses head into 2016 in much finer fettle than that which they began 2015," an economist at Markit Phil Smith said.