What is the closest airport to the center of the city it serves?

San Diego, California and SAN: about 1.5km from downtown to the airport perimeter, and just over 3 km to the main passenger terminal.

Your best bet is Eilat, Israel.

The airstrip and the terminal is literally at the city centre.

Eilat's population as of 2017 is 50,724, so it satisfies your condition of a decent-sized city.

Billy Bishop airport in Toronto is less than a 200m from the Toronto central waterfront. How far it is from the city center depends mostly on how you define city center.

It's 1,5 km on foot (1,23 km in a straight line) from the Gibraltar International Airport terminal to the beginning of the Main Street. Although Gibraltar's population is officially only 32k, so I don't know if it's decent-sized enough for the purpose of the question ;)

Google Maps

In Pisa, Italy, there are less than 3 km between the airport terminal and the Leaning Tower of Pisa which is on the opposite side of the city.

In Ponta Delgada (São Miguel Island, Azores) there are 4 km between "city center" and the terminal which is on the furthest end of the runway. But it's about 1,5 km to the closest end of the runway.

There are a lot of airports in Western Europe which are no far than 6 km from the city center they serve. Salzburg - about 3 km, Olbia (Italy, Sardinia) less than 4, Innsbruck - about 4 km, Bremen - about 4 km, Friedrichshafen - less than 5. Airports of Antwerpen, Basel, Düsseldorf, Eindhoven, Florence, Nürnberg, Rotterdam, Palermo and many other cities are within 6 km from the city center.

It's surprisingly close for a European city, but Reykjavík Domestic Airport (RKV) is only 2.1km from the city center; the closest edge of the airport only half that distance.

Do not confuse it with Keflavík International Airport (KEF) which most people use to fly to Reykjavik; that lies at a comfortable 50km distance.

Honorable mention should be given to Mexico City. The international airport is about 5km from the city center. That is not the shortest distance, but Mexico City is one of the bigger cities of the world. The airport is completely surrounded by urbanized greater Mexico City. It wasn't that way when the airfield was first built.

Ronald Reagan Washington National Airport is maybe six kilometers from the White House, and less than half that to the Pentagon (which is in Arlington, as is the airport).

Taipei's smaller, more central and less internationally known airport (Taipei Songshan Airport) is very, very close to the city center at about 3km - 4.5km depending on what you consider the center of Taipei.

It's also the only city on this list that is a megacity and a capital of a country.

https://goo.gl/maps/YcQjhkbsn7ddAMgaA

Belize City's (57k inhabitants) Municipal airport is literally downtown.

Boston Logan airport's Terminal C, the farthest from the center of the city, is 3.4 km (2.1 miles) as the crow flies from Downtown Crossing.

I like the previous answers as well, but I'd put Gibraltar as a contender. It literally bisects an urban thoroughfare and I walked from the major airplane to my hotel with my luggage and could walk from the airport to any location in the city - if you happen to not be able, there are taxis, etc.

Fukuoka International Airport (FUK) serves Fukuoka, a major city in Japan with a population of around 2.5 million people. It's less than 3km to the city center, and is only 6 minutes (2 stops) with the subway. Can't think of any other airport in the world where you can be in the city center in that amount of time with public transportation. The international terminal (southwest of the runway) is even closer at just above 2km.

CXH must be it. Not sure what the center of Vancouver even is, but it's 500m from Waterfront Station which is the most important transit hub in Vancouver and 800m from Vancouver City Center station which, I guess, could be called the center of the city? It is a floatplane terminal, sure, but it has domestic and international flights both. There are more than 50000 people living in Vancouver. Even the downtown peninsula has more according to census data.

It also has the distinct advantage of being a 30 minute walk from my home :)

Las Vegas airport perimeter to the Strip is about two blocks/1.5km

https://www.google.com/maps/dir/36.1144423,-115.1639602/36.1007993,-115.16383/@36.1088893,-115.1689542,15.25z/data=!4m2!4m1!3e2

St. Martin, population 32,000, has the airport right on the beach.

Image source

People have been hurt due to this.

Allow me to nominate Burbank (Bob Hope) Airport. Burbank is a suburb of Los Angeles with over 100,000 population in its own right. The airport is downtown, to the extent downtown is meaningful with sprawl.

Not the closest, but from Da Nang International Airport (DAD) to what OSM calls the city center is only about 2.1 km on foot. Google map couldn't point out the true center but you'll need a walking distance of 2.5 km to the central railway station, 2.5 km to the central Dragon bridge and 3.2 km to the central market

Da Nang is the largest city in central Vietnam, and the 3^rd largest city in Vietnam with a population of more than 1.2 million people in 2019. It's dubbed "the most livable city in Vietnam"

It's probably not the sort of airport you're thinking of, but in Washington State, it's 800 meters from Kenmore City Hall to the docks of Kenmore Air Harbor. And yes, it does have an airline operating regularly scheduled flights: Kenmore Air.

(Also in the Seattle area: the docks of Lake Union Seaplane Terminal are about 1100 meters from the Space Needle, and for a more traditional airport, the perimeter of Renton Municipal is about 1300 meters from Renton City Hall.)

Cuenca, Ecuador. Population >300k - the SW end of the runway is closer to downtown than it is to the NE end of the runway.

Taillinn, Estonia (TLL) is remarkably close to the city; depending on how you count, it's about 6-7 KM to the city center.

London City Airport (LCY) and Berlin Tempelhof Airport (THF) should both be in the running, though Tempelhof is closed now.

Honorary mention

Trivandrum International Airport, which serves Thiruvananthapuram, Kerala, India is just 4 KM walking distance from literal city center, Kerala Secretariat. Straight line distance will be lot less. And the airport marks the western edge of the city.

Does Taipa count as "downtown Macao"? If so, the Macau International Airport's air terminal is maybe about 1-2km straight-line distance from the center of Taipa. By foot or by car, the distance is closer to 3km, but still very short.

Now, "cheating" time:

• the New Chitose International Airport (CTS) is less than 5km to the downtown of Chitose, Hokkaido (pop. 97,458), although most passengers arriving at CTS are heading towards Sapporo;
• the Komatsu Airport (KMQ, co-located with the JASDF Komatsu Base) is less than 4km away from the city of Komatsu, Ishikawa (pop. 106,531), although most passengers access Kanazawa.

Copenhagen Airport Kastrup (pop 602k) is 6-7km from the city centre.

Lisbon Airport Portela (pop 504k) is also around 6km from the city centre.

And both are capital cities.

Some more:

Bilbao Airport, Spain (pop 345k) is also only around 4km from the city centre as the crow flies, but due to mountains it's a 12km drive.

Macapá International Airport, Brazil (pop 369k) is less than 2km from the city centre (I've personally walked to this airport)

Val de Cans International Airport (Belém, pop 143k), Brazil is around 4km from the city centre.

Similar, Fortaleza (population 862k) have the airport inside the city, around 2km from the city centre.

And the airport of Leticia, Amazonas (population ~42000) is 200 metre from the city centre. The city is intertwined with Tabatinga in Brazil (population ~59000), who also have it's own airport roughly 800 metres from the city center.

Dushanbe, Tajikistan (pop ~770k) also have a airport in the city. Now I disagree with what Google marks as the city centre, but if we're to use it, then the airport is only 12-1500 metres from the centre.

And staying in the same region, Bishkek's international airport (Kyrgyzstan, pop 900k) is about 2km from the city centre.

Basically anywhere where landmass isn't an issue, it seems airports are very close to the city centre, as building the airports and metropolisation of the cities often happened at the same time.

Marrakesh Menara Airport (RAK) is 4-6 km away from the center.

Mashable has an article on living on an aircraft carrier. In it they quote a high-ranking Navy officer:

Rear Admiral Batchelder described a carrier as being like a “small floating city," of which the captain is essentially mayor.

If you'd consider a Nimitz-class aircraft carrier a city, then you'd get really close. The schematic below by the U.S. Naval institute shows the inside of a (future) Royal Navy aircraft carrier:

As you can see, the officers' quarters are only just below the landing deck on the right. The galleys seem to be mostly in the middle of the ship (according to the image). In conclusion, you could say the distance from the landing strip to the city is negligible, in the order of metres.

The airport in Billings, MT (BIL) is on a ridge overlooking downtown. The east end of the runway is barely a mile from city hall and the courthouse (which is where many places consider the "center of town" for purposes of mileage signs, etc).

Today I also learned that Syktyvkar (pop. approx. 240k people) airport is located practically downtown of this northern city, within 1km of its central square, counting from terminal building.

And nobody is tearing down or closing this one, compared to many other entrants of this page.

I think it must be Chhatrapati Shivaji Maharaj International Airport, commonly known as Mumbai International Airport (BOM). It also holds the record of being the busiest single runway airport by aircraft movements (1000+ aircraft movements in 24 hours). Also it serves as the only airport as of now for a megacity which has a population of about 20 million. It can also serve large aircraft like the A380 despite being located right in the heart of the city unlike other airports which are situated very close to the city centre (like London City Airport)