In an email to Kickstarter's members, chief executive Yancey Strickler made it clear that "no credit card data of any kind was accessed by hackers".
Strickler said that law enforcement officials contacted Kickstarter last week and alerted the company that hackers had "sought and gained unauthorised access to some of our customers' data".
Kickstarter immediately closed the security breach and began strengthening security measures throughout the platform's system, he added.
Strickler said: "While no credit card data was accessed, some information about our customers was. Accessed information included usernames, email addresses, mailing addresses, phone numbers, and encrypted passwords,"
He claimed that "actual passwords" were not revealed, but it would be possible for "a malicious person with enough computing power" to guess and crack an encrypted password, particularly a weak or obvious one.
Kickstarter is urging its users to change their passwords on their accounts, and any other account that uses the same password.
Strickler apologised to members, stating: "We're incredibly sorry that this happened. We set a very high bar for how we serve our community, and this incident is frustrating and upsetting.
"We have since improved our security procedures and systems in numerous ways, and we will continue to do so in the weeks and months to come. We are working closely with law enforcement, and we are doing everything in our power to prevent this from happening again."