Copulatory behaviours stand as cornerstones of sexual selection, yet they remain mysterious in many species, especially in bats. An extremely large penile erection with a heart-shape terminal swelling has been observed in the serotine bat (Eptesicus serotinus). The engorgement of the erectile tissues could take place after penetration to generate a copulatory lock. Alternatively, the erected penis, long and mobile, could be used to pass by the protective tail membrane to reach the vulva. In the latter scenario, the penis may however not be able to penetrate the vagina. In order to find out which of the two scenarios occurs, we investigated the morphology of female and male genitalia of E. serotinus. We found that the vagina is more than seven times shorter than the erected penis, supporting that the penis is used as a copulatory arm rather than an intromittent organ. We could confirm our hypothesis with direct observations of copulations. During mating, the male grasps the female in a dorsoventral position. The male probes the female’s ventral part with its erected penis until the penis is tightly pushed against the vulva. Afterward, the pair stops moving and can hold the position for several hours. In addition, the timing of copulations, which exhibits a peak in October, implies that visits to swarming sites throughout the year may serve other functions besides mating. This research reveals a novel copulatory behaviour that has not been previously documented in mammals and sheds light on the poorly understood area of bat reproduction.