The Macmillan Cancer Support study shows the detrimental impact the emotion can have, leaving patients housebound, unable to feed themselves properly and likely to drink more.
The charity warned of a looming "loneliness epidemic" as the number of people with cancer is expected to double in the next 20 years.
The Ipsos Mori research on behalf of Macmillan found Scots patients with cancer were lonelier than those in the rest of the UK.
According to the poll, 31% of Scots (58,000 people) with cancer were lonelier since diagnosis, compared to 16% in Wales and 21% in England.
Elspeth Atkinson, director of Macmillan in Scotland, said: "Loneliness is affecting the lives of thousands of cancer patients in Scotland and this research shows the impact it can have is huge.
"We want to be there for all cancer patients who need us and have a range of services to help, including our information and support services, a support line and an online community.
"These services are a lifeline to people affected by cancer. We need the NHS, policy makers and local authorities to continue to work with us to provide these vital services to ensure no one faces cancer alone."
The study found that across the UK, lonely patients were three times more likely to drink more alcohol than they usually did. This affected an estimated 13,000 lonely people with cancer in Scotland.
They were almost five times more likely to have not left the house for days (affecting an estimated 39,000 people in Scotland) and almost three times more likely to have problems sleeping (45,000).
Lonely cancer patients across the UK were also five times more likely to skip meals, (an estimated 22,000 cancer patients in Scotland), while they were almost eight times more likely to have a poor diet. This affected an estimated 27,000 lonely Scots patients.