CD9 has been reported to play a role in tumor metastasis suppression. However, it is not fully understood how CD9 affects the hematogenous spread of tumor cells. To clarify a new mechanism (or mechanisms), we generated HT1080 cells that had been transfected with a CD9-expressing plasmid. Ectopic expression of CD9 in HT1080 cells actually reduced their metastatic ability. CD9 expression reduced lung retention and platelet aggregation activity of the transfectants. Because HT1080 cells express the metastasis-promoting, platelet aggregation-inducing factor Aggrus/podoplanin on their surface, we examined the relationship between CD9 and Aggrus. We discovered that CD9 formed a complex with Aggrus via transmembrane domains 1 and 2 (TM1 and TM2) of CD9. Investigation of the interaction revealed that each CD9 and Aggrus interacted homophilically, and that they colocalized in low-density membrane fractions. Deleting TM1 and TM2 attenuated the ability of CD9 to interact homophilically or to localize in low-density membrane fractions. The expression of CD9-wild-type (WT), but not CD9 lacking TM1 and TM2, attenuated the platelet aggregation and metastasis induced by forced expression of Aggrus in CHO cells. Therefore, CD9 may act as a metastasis suppressor, at least in part, by neutralizing Aggrus-mediated platelet aggregation.