Some 8 per cent of journeys to and from the terminal were by tram at beginning of the year, compared to around 25 per cent by bus.
The report by the Airport Operators Association (AOA) also showed trams’ share of passengers fell from 10 per cent over the previous months.
The proportion travelling by bus - most of whom take services into Edinburgh - also fell slightly.
Tram tickets cost £5.50 single and £8.50 return - which are both £1 more than Lothian Buses’ Airlink service.
One source said the fall could be connected with the launch of Uber in the capital.
Passenger watchdog Bus Users Scotland said passengers may be opting for buses because they left from nearer the terminal.
Director Gavin Booth said: “For passengers, the introduction of the trams to Edinburgh Airport in 2014 has increased choice and provided extra capacity, which is good.
“The tram and the Airlink bus partly serve the same market – certainly between Princes Street and Haymarket, and between Gyle Centre and the airport, but for the major part of their journeys they are providing an airport link for passengers living along or near two very different corridors, so both modes have an important part to play.
“We know passengers arriving at the airport from major European cities expect a rail link to the city centre – heavy rail or tramway – and will look for the tram, while many will encounter the Airlink bus first and use this.
“Both are very good services, with little difference in end-to-end times, particularly in view of the likely speeding up of the tram times. For passengers at least it is a win-win situation.”
A spokesman for Edinburgh Airport said: “We’re working hard with all of our partners to give passengers the best choice of surface transport options.
“We expect Edinburgh Gateway station, which will connect tram to the rail network, to bring a further increase in the public transport mode share of our passengers when it opens next month.”
A city council spokeswoman said: “The tram has been a popular way of travelling to and from the airport for residents and visitors, but without seeing the actual figures, we are not in a position to comment.”
The overall tram passenger total increased by 9 per cent to nearly 5.4 million in the year to May.