Police have named the 79-year-old patient who was allegedly attacked in hospital.
Hugh Drummond is in a critical condition following an incident at Wishaw General Hospital on Thursday.
Strathclyde Police said the alleged serious assault took place at around 5am.
A 45 year-old man has been arrested and is expected to appear at Hamilton Sheriff Court on Tuesday.
A health board spokeswoman said: "NHS Lanarkshire are assisting police with their inquiries into an incident which took place within Wishaw General Hospital relating to a 45-year-old male inpatient and an alleged assault on a 79-year-old in-patient."
A Strathclyde Police spokeswoman said: "A 45-year-old man has been arrested following an alleged assault on a 79-year-old man within Wishaw Hospital at 5am on Thursday 20 September."