Economists had expected the index to come in at 53.8 in May, down from 54.5 in April.
In the preliminary reading, Markit's Chris Williamson wrote: "Manufacturers reported their weakest growth since the start of 2014 in May, with the survey results adding to fears that the strong dollar is weighing on the US economy and hitting corporate earnings.
"Although falling only modestly, export sales have now dipped for two straight months, something not seen for two years and a far cry from the solid export performance seen this time last year. Overall order books are consequently growing at the slowest rate seen since the start of last year."
More to come ...