It is understood two people were on board the aircraft which was carrying salmon when it ditched in North Uist.
Police Scotland said the alarm was raised at 10.15am.
A police statement said: “As a result of the incident, the A867 is closed between Lochmaddy and Clachan Na Luib.
“The diversion route is via the A865 and motorists are advised to drive with care and that this road is not suitable for larger vehicles.”
More to follow.