Magnotta is charged with first-degree murder in the death and dismemberment of Concordia University student Jun Lin.
A Toronto judge approved the release of the document concerning Magnotta's medical history.
The letter was submitted as evidence when Magnotta was facing fraud charges in 2005.
At the time, he was going by his birth name — Eric Clinton Newman.
The letter was from a Toronto-area psychiatrist who had been treating him for five years.
The psychiatrist said he knew of two recent occasions when Newman was admitted to hospital as an inpatient.
Media fought to make document public
The letter was released after several media outlets fought for it to be made public.
A Globe and Mail reporter had been trying to obtain the letter, a public document, since July, but was rebuffed in several attempts.
Various media outlets then joined together to hire counsel to make arguments to a judge for the letter's release.
Magnotta's lawyer Luc Leclair tried to stop the letter from becoming public, but Ontario Court Judge Fergus O'Donnell ruled Tuesday afternoon that it should be released.
In his ruling, O'Donnell said the document does contain sensitive and personal medical information, but any privacy interest was forgone when the letter was filed in open court.
"Without access to the letter, the public is not in a position to engage in a meaningful assessment or debate over the appropriateness of what happened to Mr. Newman in 2005 in what is supposed to be an open and transparent court process," O'Donnell wrote.