Air France KLM carried a total of 9.2 million passengers across its networks last month, up 3.3% from the same period last year.
The airline's new chief executive Ben Smith is aiming for a better co-ordination of the Air France and KLM networks.
Air France KLM's May passenger load factor - an industry metric that measures how much of an airline's passenger carrying capacity is used - also rose 1.8 percentage points from a year earlier to 87.3%.
Earlier this week, British Airways and Iberia airlines company International Consolidated Airlines Group (IAG) also reported higher passenger traffic figures for May.
But the airline industry as a whole is facing pressures from trade wars and higher oil prices.
Air France KLM posted a first-quarter loss in May, and the International Air Transport Association (IATA) body said in June that global airlines had slashed profit forecasts for 2019 due to concerns over the trade disputes and oil prices.
The French state holds 14.3% of Air France KLM's share capital, while the Dutch state has a 14% stake.
Delta Air Lines and China Eastern Airlines also each have stakes of 8.8% in Air France KLM.