The leak follows similar breaches of passwords at professional networking site LinkedIn and music site Last.fm.
"After investigating reports of compromised passwords, we have found that a small fraction of our user base has been affected," eHarmony spokeswoman Becky Teraoka wrote in a company blog post.
Members whose passwords were affected will receive an e-mail with instructions on how to reset their passwords, Teraoka said.
The exposed data was in the form of cryptographic representations of passwords called hashes, which can be converted back to the original passwords with decoding software.
Hashes of 1.5 million eHarmony passwords were posted on a forum of a Russian password-cracking Web site called InsidePro, Ars Technica reported.
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