The final Markit/Nikkei services Purchasing Managers' Index (PMI) for Japan climbed to 52.4 in January from 51.5 in December.
A reading above 50 indicates expansion in the sector, a reading below 50 indicates contraction of activity.
The index was driven by a rise in new orders.
"January data showed positive signs for the Japanese service sector. Business activity growth accelerated to a five-month high, supported further by a solid expansion in new orders. With the headline indices remaining positive in both the manufacturing and service sectors, this bodes well for growth early on in 2016," economist at Markit, Amy Brownbill, said.