Significance The development of dental plaque biofilm requires specific and sequential molecular interactions between oral bacteria-colonizing host surfaces. Coaggregation between early colonizers is crucial to establish an environment suitable for late colonizers. Here, we describe that a surface protein in a Gram-positive bacterium that is not genetically linked to the fimbrial gene clusters hijacks a specific fimbrial polymerization apparatus to be displayed on the fimbrial tip. This tip-localized protein not only functions as the bona fide cell-to-cell adhesion factor for mediating coaggregation between the early colonizers Actinomyces oris and Streptococcus oralis but also serves as an initiator of fimbrial assembly.