The report, completed three years ago to evaluate his fitness for trial, has been posted online at Hasan's request by his former defense attorney, retired Col. John Galligan.
Trial judge Col. Tara Osborn told prosecutors the report was off-limits to them.
"You will not access, read or in any way review the full sanity report," she said after jurors left for the day Wednesday, the San Antonio Express-News reported.
"That goes for all members of the trial team," she added.
Prosecutors said they were aware the report was on the Internet.
"We will do our best to avoid it," Col. Steven Henricks said.
Hasan told Osborn he had intended for the report to be made public.
"I effectively waived my privilege," he said.
Hasan acknowledged at the start of his trial last week he was the gunman who killed and wounded service members at a processing center at Fort Hood.
He is charged with 13 counts of premeditated murder and 32 counts of attempted murder.