One forum alone accounted for almost half of the breached data -- a little under 13 million records; the other two forums make up over 12 million records.
The databases were stolen in early August, according to breach notification site Leaked Source.com, which obtained a copy of the databases.
It's also not the first time that has suffered a breach this year.
The hackers were able to obtain usernames, email addresses, scrambled passwords, and birthdays.
Some of the forums allowed the hackers to also obtain IP addresses (which could be used to determine location) and phone numbers.
Over 25 million accounts associated with forums hosted by Russian internet giant have been stolen by hackers.
Two hackers carried out attacks on three separate game-related forums in July and August.
The group said that the most common four passwords were some combination of "123456789", which in part made it easier to determine a significant portion of the leaked passwords.
The hackers' names aren't known, but they used known SQL injection vulnerabilities found in older v Bulletin forum software to get access to the databases.
An analysis of the breached data showed that hackers took 12.8 million accounts from ru; a total of 8.9 million records from ru, and 3.2 million accounts from ru.
This is the latest hack in a long line of similar attacks on out-of-date and unpatched forums with widely known and glaring security flaws.
Many of Mail.ru's forums ran versions of v Bulletin software dating back to early-2013.
A member of the Leaked Source group told me that about half of the passwords -- around 12 million -- were easily cracked using readily available cracking tools.
That's because, according to the group's blog post, the sites "all used some variation of MD5 with or without unique salts", an algorithm that is considered insecure by today's standards.