Through security
Every TSA / airport security station allows watches through, on the wrist or in your carry-on. Mechanical watches contain no batteries (no concern), no liquids, and no flammables. Quartz watches contain a small lithium-ion or silver-oxide battery (under the 100 Wh / 2g lithium limit by orders of magnitude, no concern). The X-ray scanner is high-frequency electromagnetic radiation, not a magnetic field; X-rays do not affect mechanical watches. Some travellers prefer to drop their watch into the carry-on and put it back on at the gate; others walk through with it on. Both are fine.
Cabin pressure
Cabin pressure on a commercial flight is regulated to roughly 0.75 atmospheres (equivalent to 8,000 ft elevation). Any watch with a water-resistance rating of 30m or higher is engineered to hold a much wider pressure differential than this; cabin pressure is essentially a non-event. The exception is open-back / display caseback watches with weak crystals (acrylic / hesalite), where extreme thermal cycling could theoretically cause issues. In practice this is a non-issue; nobody loses a watch to cabin pressure. For more on water-resistance ratings, see our water resistance guide.
Magnetism: the small real risk
The walk-through metal detector at airport security uses an oscillating magnetic field to detect metal. Field strength is regulated, typically under 600 gauss at the closest approach, and your wrist is rarely closer than 30cm to the coil. Modern watches with silicon hairsprings or anti-magnetic certification (Omega Master Chronometer 15,000 gauss, Rolex Milgauss 1,000 gauss, IWC Ingenieur) shrug it off completely. Older mechanical watches with steel hairsprings can pick up some magnetisation from the gate, occasionally leading to noticeable rate gain afterwards (5-30 sec/day fast). Easy fix: any watchmaker can demagnetise the watch in 30 seconds for under CHF 50. The risk is not zero, but small enough not to worry about.
Where the real risk is
Theft. Watch theft has risen sharply in major travel hubs (London, Paris, Madrid, Milan, New York). Most theft happens on the airport floor: distraction theft at the security tray (you put the watch in a bin, it disappears), at the rental car counter, in airport lounges, and crucially in airport parking lots. Some best practices: do not put a Rolex in a security tray; either keep it on your wrist (allowed) or in your zipped carry-on; tip-of-the-iceberg watches (Daytona, Royal Oak, Sub Date) are most-targeted. Avoid wearing your most-expensive watch on travel days through known hot-spot airports.
In-flight: comfort and the watch winder myth
A mechanical watch on your wrist during a flight is fine; sleeping on a flight with a watch on the same wrist as your face cushion can scratch the crystal. If you remove it for sleep, put it in a soft pouch inside a zipped pocket of your carry-on, never in a seat-back pocket (the most-frequently-stolen-from spot on aircraft after the overhead bin). Watch winders are not necessary in flight; a modern watch holds 60-72 hours of power reserve and a typical international flight is 6-12 hours.
Special cases
Smart watches (Apple Watch, Garmin, Galaxy): keep them in airplane mode during flight per FAA / EASA rules; cellular data and Bluetooth normally disable automatically when "in-flight" mode is detected. Quartz watches with batteries: no battery limit issue (the cells are tiny). Expensive collector pieces in checked baggage: insurance industry strongly advises against. Most travel insurance and homeowner policies do not cover watches lost from checked baggage; carry-on or on-wrist only. Customs declarations: most countries require declaration only above a value threshold (typically USD 10,000+); a single luxury watch under that limit usually does not need declaration, but check your destination's rules.