The whole of the Western Isles will go into lockdown from midnight.
Health secretary Jeane Freeman said the move to level four was necessary due to a rise in coronavirus infections across entire council region.
All of mainland Scotland is currently in full lockdown, with non-essential shops closed and they ‘stay at home’ message in place.
Freeman said: “When the situation demands it, we must act quickly and decisively to safeguard health, save lives and protect the NHS.
“As we said we would, we have kept the island areas under review. The advice of our clinical advisers is that due to a rising trend of infections, lockdown is now necessary in the Western Isles.
“Discussions are underway to offer support and ease the pressure on local health services.
“It is essential that we further limit interaction between different households to stem the spread and bring the situation back under control, while we vaccinate more people.”
The Scottish Government said six new cases were reported in the Western Isles on Friday and nine on Thursday.
Those cases follow a previous rise – including a community outbreak on Barra – that has placed a significant strain on hospital capacity in the Western Isles health board area.
It also said the hospital in Stornoway, on the Isle of Lewis, has now reached full capacity.
Some non-urgent procedures will be rescheduled but the hospital will maintain a full emergency service.
Council leader Roddie Mackay said: “The designation of the Western Isles as a level four Covid-19 area is disappointing but not entirely unexpected given the recent rise in cases and the developing situation at the Western Isles Hospital in Stornoway.
“Our communities continue to make stringent efforts to protect themselves and others and it is imperative, now more than ever before, that we maintain and indeed – wherever possible – increase these efforts.”
Earlier this month, Barra and Vatersay moved into level four of the government’s coronavirus framework.
The islands join the whole of mainland Scotland, as well as Skye, which are under almost a full lockdown with schools, non-essential shops, gyms, salons and most hospitality venues closed.
Those lockdown restrictions, which apply to all level four areas, are in place until at least the middle of February.
Scotland has recorded 70 more deaths from coronavirus in the past 24 hours, according to official figures.
The death toll under this measure – of people who first tested positive for the virus within the previous 28 days – is now 6040.
A further 1155 positive cases were also reported by the Scottish Government.