In a blog post, Reddit's founding engineer Christopher Slowe characterised the attack as "serious", but added that the attacker did not gain write access to Reddit's systems.
"...they gained read-only access to some systems that contained backup data, source code and other logs," he said.
The site had learnt of the breach on 19 June, Slowe said, adding that a compromise of some users' accounts had taken place between 14 and 18 June.
The old database copy contained user data between the launch of the site in 2005 and 2007, including account credentials, email addresses and all content, both public and private.
Email digests sent by Reddit between 3 and 17 June were also accessed, Slowe said, adding, "The digests connect a username to the associated email address and contain suggested posts from select popular and safe-for-work subreddits you subscribe to."
The company has reported the breach to law enforcement, was informing those whose credentials had been accessed and taken steps to ensure that access to the site is made more secure.
"If your account credentials were affected and there’s a chance the credentials relate to the password you’re currently using on Reddit, we’ll make you reset your Reddit account password," Slowe said.
"Whether or not Reddit prompts you to change your password, think about whether you still use the password you used on Reddit 11 years ago on any other sites today."
Commenting on the breach, Jessica Ortega, product marketing associate and member of web security firm SiteLock's research team, said: "With the breach of encoded passwords and usernames, attackers could potentially gain unauthorised access to affected Reddit user accounts.
"Additionally, the disclosure of email addresses and their connected Reddit usernames could potentially mean attackers can identify and dox (release the identifying information of) users who rely on Reddit for discussing controversial topics or posting controversial images.
"It is recommended that all Reddit users update their passwords. Users should also practice good cyber hygiene habits and avoid reusing their password on any other website. For those concerned about the potential fallout from being doxed, they can also change the email address associated with their username."
Tyler Moffitt, a senior threat research analyst at security firm Webroot, said: "While Reddit’s use of SMS-based authentication is popular and much more secure than using a password alone, it’s widely known to be vulnerable to cyber criminals who have hacked many celebrities using this method.
"In this type of attack, the phone number is the weakest link. Cyber criminals can steal a victim’s phone number by transferring it to a different SIM card with relative ease, thereby getting access to text messages and SMS-based authentication.
"For example, a cyber criminal would simply need to give a wireless provider an address, the last 4 digits of a social security number, and perhaps a credit card to transfer a phone number. This is exactly the type of data that is widely available on the dark web thanks to large database breaches like (that at) Equifax.
"While it seems that the cyber criminals only have read-access to this data, I’m glad that Reddit is now moving to a token-based two-factor authentication model, which provides a greater layer of security.”