New Monteith House in Carstairs, South Lanarkshire, which has 25 residents, has been told it must address issues identified by a recent inspection or face being de-registered.
The Care Inspectorate said "significant progress" must be made in meeting residents' nutritional needs and ensuring appropriate staff numbers.
The home must also demonstrate that staff are fit to be employed and ensure residents' care plans are reviewed and updated.
A spokesman for the regulator said: "There were four areas of serious concern raised by our inspectors and we have informed the services of the action it must take to ensure it meets the standards we require.
"We will be monitoring this service closely and will be inspecting again soon."