Melissa Ann Shepard, 80, was freed in March after a jail term for drugging her new husband.
But police said she was a high-risk re-offender and set a number of conditions, including not going online.
She used a computer on Monday to access the web at a library in Nova Scotia.
Police have warned elderly men looking for love to steer clear of Shepard, with the internet flagged up as a possible threat.
In addition to not using the web, she is also required to abide by a curfew and inform police before starting a romantic relationship.
Her prior convictions include the manslaughter of her second husband in 1992, whom she drugged and ran over twice in a car.
Her third husband died in 2002 shortly after they were married. No charges were brought in connection with his death.
Shepard was sentenced to five years in prison in 2005 for seven counts of theft from a man in Florida whom she met online and with whom she lived for a month.
She was also convicted of stealing from another man Alex Strategos, 73. Mr Strategos believes Shepard drugged him as well, although she was not charged.
Shepard was charged again last October after her latest husband, Fred Weeks, 75, fell ill from the effects of tranquilisers mixed into his coffee on board a ferry from Nova Scotia to Newfoundland on their honeymoon the previous month. He survived.
A Canadian woman dubbed the "Black Widow" after a series of convictions involving male partners has been charged with breaking the terms of her release.