The HSBC/Markit Services Purchasing Managers' Index (PMI) for China declined to 51.8 in June from 53.5 in May. It was the lowest level since January.
A reading above 50 indicates expansion in the sector.
"In the service sector, business activity, new orders and employment all expanded at slower rates, while optimism towards the business outlook also moderated," an economist at Markit, Annabel Fiddes, said.
The new business sub-index dropped to 52.2 in June, an 11-month low, from 54.4 in May, while the employment sub-index decreased to its lowest level in three months.