Managing director Mark Johnston said the total was forecast to fall to 9.1m this year from 9.7m in 2018 and 9.9m in 2016.
It follows Ryanair dramatically slashing its routes from Scotland's second busiest airport last October from 23 to three, although this will increase slightly to seven this summer.
Glasgow's decline comes in stark contrast with rival Edinburgh, which added 900,000 passengers last year to increase its total to 14.3m.
This is likely to rise further with new routes such to Philadelphia in April and Boston in May in addition to new European flights.
Glasgow said it would return to growth next year on the promise of more flights being secured from discussions it is having with airlines, which could include Ryanair.
The airport will see some expansion this year, such as the launch of the Airbus A380 - the world's largest passenger plane with more than 500 seats - on Emirates' Dubai route in April, in a Scottish first.
However, Mr Johnston said that and other growth, such as Jet2 basing a seventh aircraft at the airport this summer, would not offset set the downturn.
Other losses have include its route to Philadelphia, and winter flights to New York.
Mr Johnston told a Glasgow Talks event organised by Glasgow Chamber of Commerce today: "We forecast further reductions [this year.
"We will lose around a million passengers."
He said Ryanair had blamed its cuts on the Scottish Government's failure to make a planned 50 per cent cut air passenger duty, which is currently £13 per passenger on European flights departing from the UK.
The routes cut were to Alicante, Frankfurt, Riga, Berlin, Lanzarote, Sofia, Bydgoszcz, Las Palmas, Stansted, Carcassonne, Lisbon, Valencia, Chania, Madrid, Warsaw, Charleroi, Malaga, Zadar, Derry, Palanga,
The airline will reinstate Alicante, Charleroi, Malaga and Warsaw from March or April.
Mr Johnston said the reductions showed air passenger duty was now causing airlnes to reduce services.
He said: "We are now seeing a reversal rather than it being an inhibitor to growth".
However, the airport chief said the outlook if Brexit goes ahead was looking better for flying.
He said there would be no mass grounding of aircraft, as had been previously feared.
But Mr Johnston admitted there had been a "slight deterioration" in consumer confidence, with flight bookings from the UK to European Union countries this summer down by 12.2 per cent.