During a search of a property in Degersheim last week police seized 280 weapons, over 100,000 rounds of ammunition and 1.3 million francs in cash, St Gallen police said in a press release.
Among the weapons haul were submachine guns, grenades, pistols and rifles.
The man was suspected of illegally selling arms to Austria and has been the subject of a joint investigation by Austria and Swiss authorities for the past five months.
He is now in custody, accused of threatening public safety and violating the weapons act.
The investigation is ongoing.