The index grew for the 88th consecutive period, data revealed on Wednesday. Economist polled by Thomson Reuters had forecast the index to only reach 55.8, but a jump in new orders boosted growth higher.
The non-manufacturing index hit 55.2 in March, down from 57.6 in February. This represented continued growth in the non-manufacturing sector but at a slower pace.
A reading above 50 on this index indicates expansion in the service sector, and a reading below 50 indicates contraction.