In this study, we elucidated the formation process of an unconventional biofilm formed by a bacterium autoagglutinating through sticky, long, and peritrichate nanofibers. Understanding the mechanisms of biofilm formation is essential to control microbial behavior and improve environmental biotechnologies. Acinetobacter sp. Tol 5 autoagglutinate through the interaction of the long, peritrichate nanofiber protein AtaA, a trimeric autotransporter adhesin. Using AtaA, without cell growth or extracellular polymeric substances production, Tol 5 cells quickly form an unconventional biofilm. The process forming this unconventional biofilm started with cell-cell interactions, proceeded to cell clumping, and led to the formation of large cell aggregates. The cell-cell interaction was described by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory based on a new concept, which considers two independent interactions between two cell bodies and between two AtaA fiber tips forming a discontinuous surface. If cell bodies cannot collide owing to an energy barrier at low ionic strengths but approach within the interactive distance of AtaA fibers, cells can agglutinate through their contact. Cell clumping proceeds following the cluster-cluster aggregation model, and an unconventional biofilm containing void spaces and a fractal nature develops. Understanding its formation process would extend the utilization of various types of biofilms, enhancing environmental biotechnologies.