Western Health said it fired the employee after discovering that 1,043 people had their privacy breached.
"Western Health would like to apologize to each patient who has had their privacy breached," Dr. Susan Gillam, the health board's CEO, said in a statement.
"We take our responsibility as the custodians of an individual's personal health information very seriously and with the utmost care and concern. Privacy breaches such as this one are extremely unfortunate."
Gillam said Western Health is contacting the patients of the breach and the province's privacy commissioner has been notified.
This marks the third time this year and the second time in a week that a health board in the province has reported a privacy breach.
Last week, Eastern Health said it fired one of its employees after someone inappropriately accessed the medical records of 122 patients.
In March, three urine samples and eight vials of blood with patient names on them were discovered near a busy St. John's road. Eastern Health said the biomedical waste originated from the emergency department of the Health Sciences Centre and was inappropriately disposed of in regular garbage that was sent to a landfill.