European Travel Commission (ETC) data shows the number of people hoping to travel to the Mediterranean region from June to November has already fallen 10% compared to last year when scorching weather led to droughts and wildfires.
Destinations like the Czech Republic, Denmark, Ireland and Bulgaria have meanwhile seen a spike in interest.
A report by the trade body also shows 7.6% of travellers now see extreme weather events as a major concern for trips between June and November.
Demand for travel has soared again this summer as tourists leave behind years of pandemic restrictions, and travel companies say the heat hasn't caused many cancellations - yet.
Britons in particular have booked fewer holidays at home and more in the Mediterranean, often many months in advance, as they continue to crave post-lockdown beach escapes, said Sean Tipton of British travel agent group ABTA.
But that balance could shift as heatwaves are set to become more gruelling. Scientists have long warned that climate change, caused by CO2 emissions from burning fossil fuels, will make weather events more frequent, severe and deadly.
Meteorologists predict that temperatures in the coming week may surpass Europe's current record of 48.8 degrees Celsius (119.84 Fahrenheit), set in Sicily in August 2021, raising fears of a repetition of last year's heat deaths.
Stories of tourists being airlifted off Italian beaches or ferried away in ambulances from Athens' Acropolis have flooded European media in recent weeks.
Tourists in Rome told Reuters they would think twice about booking a trip there again in July as they struggled to drink enough water, stay cool and find air-conditioned spots to rest.
That's bad news for Italy's economy, which thrives on busy summer traffic.
Italy's Environment Ministry warned in a report this year that foreign tourists would in the future travel more in the spring and autumn and choose cooler destinations.
Some hope that the change will simply be a shift in traffic, not a reduction.
In Greece, where international air arrivals were up 87.5% year-on-year between January and March, overcrowding in the summer has plagued tourist hot spots like the island of Mykonos.
Increased travel in the winter, spring and autumn months could ease that problem and make up for a potential summer slowdown, according to the Greek environment ministry.
Greek authorities closed Athens' ancient Acropolis during the hottest part of the day on Friday to protect tourists.
In Spain, high vacation demand is expected in coastal destinations in the north of the country and on Spanish tourist islands, where summer temperatures tend to be cooler, according to a report from the national tourism association Exceltur.