The officers' home addresses, email addresses and passwords were posted online.
The California transit agency has come under fire after turning off cell phone service in its stations last week to stop a potential protest.
The hacker group Anonymous announced the breach on Twitter and published the address of the website where the officers' information could be found. It did not immediately claim responsibility for the hack as it did when it broke into BART's marketing website last week and released the personal information of more than 2,000 customers.
BART Police Deputy Chief Daniel Hartwig said his office "has been made aware of the breach" and referred inquiries to the BART Police Officers Association.
Union president Jesse Sekhon didn't immediately return a phone call. The union's website was disabled later Wednesday.
BART blocked the signals last Thursday to disrupt a planned protest against police brutality. BART's actions prompted a Federal Communications Commission investigation.
The Associated Press contributed to this report.