But a number of individual US states allow the practice.
Draft regulations to be released on Saturday will spell out who will be authorised to grow and smoke the drug - victims of terminal illnesses and chronic conditions such as cancer, Aids, multiple sclerosis and epilepsy.
They will include tight restrictions, including criminal record checks for people whose doctors prescribe marijuana and limits on the number of plants a patient may grow.
The regulations will help solve an existing problem in Canadian law - that even people who are legally allowed to use cannabis for medical purposes must resort to criminal purchase to get it.
Clarifying procedures
Canadians have been able to apply for medical use of marijuana since 1999, but only 220 people have been licenced to use it so far.
Mr Rock said a licence to grow marijuana would be issued to patients, or their designated representative if their medical condition did not allow them to cultivate it.
"Canada is acting compassionately by allowing people who are suffering from grave and debilitating illnesses to have access to marijuana for medical purposes", he said on Friday.
US concerns
There have been concerns about the policy in the United States, Canada's powerful neighbour to the south, where the Justice Department opposes medical use of marijuana.
But a number of individual US states allow the practice.
Mr Rock denied that the new regulations were a step towards decriminalising cannabis.
"We've had medical access to morphine and heroin for a long time and it hasn't been the thin edge of the wedge for legalising those drugs", he said.
In December, Canada licenced a private company, Prairie Plant Systems, to grow marijuana for medical use.
It is thought that Mr Rock is considering eventually relying on such private companies to provide medical marijuana so that patients will not have to grow it themselves.
Asked two years ago if he had used marijuana, Mr Rock - who spent a day with John Lennon in Ottawa in 1969 - said: "Not for medical purposes."