The incident occurred around 8:30pm at Gate 81 of the International Passenger Terminal, police reported.
Phakhanat Dumlak, an officer responsible for baggage checks, was alerted by Ainah Pango, an X-ray image officer, who noticed a suspicious object resembling a firearm during a routine scan.
The authorities were informed, and a subsequent search of the suitcase confirmed the presence of a short-barrel handgun, an ASTRA cal 6.35 a25 model.
The Swiss man, Kurt Fritz Loliger, was detained and handed over to Sakhu Police.
At last report, Mr Loliger was held for further investigation.