It says the online retail giant used EBay's messaging system to recruit its sellers.
In the lawsuit filed on Wednesday, EBay says Amazon representatives messaged sellers to get them to sell on Amazon's site, which EBay says violates its user agreement.
Amazon representatives spelled out their email addresses and asked EBay sellers to talk on the phone in order to evade detection.
Amazon declined to comment on the lawsuit.
Third-party sellers have become an important part of Amazon's growth.
Last year, more than half the items sold on Amazon were from third-party sellers.