The trial was carried out by a team at the Memorial Sloan Kettering Cancer Centre (MSK) in New York.
Immunotherapy harnesses the body's own immune system against cancer.
The MSK clinical trial was investigating for the first time if immunotherapy alone could beat rectal cancer that had not spread to other tissues, in a subset of patients whose tumour contains a specific genetic mutation.
These patients had tumours with a specific genetic makeup known as mismatch repair-deficient (MMRd), or microsatellite instability.
Between 5pc and 10pc of all rectal cancer patients are thought to have MMRd tumours, including all the patients in the MSK clinical trial. Around 45,000 Americans a year are diagnosed with rectal cancer.
In every case on the trial, the rectal cancer disappeared after immunotherapy, without the need for the standard treatments of radiation, surgery or chemotherapy, and all of the patients have been cancer-free for up to two years.
"It's incredibly rewarding," said co-lead investigator Dr Andre Cercek. "To get these happy tears and happy emails from the patients in this study who finish treatment and realise, 'Oh my God, I get to keep all my normal body functions that I feared I might lose to radiation or surgery'."
Sascha Roth was preparing to go to New York, where she was scheduled to undergo weeks of radiation therapy for rectal cancer, when she received a phone call from MSK medical oncologist Dr Cercek that changed everything.
Dr Cercek told Ms Roth, then 38, that her latest tests showed no evidence of cancer after she had undergone six months of treatment as the first patient in the trial.
"Dr Cercek told me a team of doctors examined my tests," Ms Roth said. "And since they couldn't find any signs of cancer, Dr Cercek said there was no reason to make me endure radiation therapy."
While the study is small so far, the results are so impressive they were published in The New England Journal of Medicine and featured recently at the nation's largest gathering of clinical oncologists.