Author Summary Retroviruses such as the human immunodeficiency virus are known to spread much more efficiently under conditions of direct cell–cell contact as compared to cell-free conditions. How cell–cell contact stimulates virus spreading is poorly understood. In this study, we apply four-dimensional imaging (3D space over time) of a model retrovirus to directly monitor and quantify the events of assembly, release, and transmission of individual viral particles in real time in living cells. Our work reveals that after contacts are established between virus-producing cells and uninfected target cells, the majority of virus particle assembly is initiated at sites of cell–cell contact. The ability of the virus to direct assembly of its particles towards sites of cell–cell contact is dependent on the presence of the cytoplasmic tail of the viral envelope glycoprotein. When this cytoplasmic tail was deleted, virus assembly at cell–cell contacts was no longer enhanced. This study contributes to an emerging model in which several steps of the viral life cycle are efficiently coordinated at sites of cell–cell contact, thereby promoting the spreading of viral infection to neighboring cells.