Ultraviolet (UV; wavelengths: 280–400 nm) colouration has been shown to be an important visual signal but has not been studied in conjunction with other signals such as vibratory signals previously. Here we investigated multimodal signal function in the visual and substrate-borne vibratory modalities of the UV-ornamented jumping spider Cosmophasis umbratica, in which the importance of UV colouration in courtship displays has been demonstrated. We first described vibratory signals produced by courting males. We found that while vibratory signals mainly consist of palp drumming and abdomen thump, amplitude of abdomen thump shows a high variance and is positively correlated with body mass. This suggests that abdomen thump as a vibratory component may be condition-dependent. To examine whether the vibratory and UV signal function as backup to each other in a variable environment (efficacy-based backup hypothesis), we used a fully crossed 2 × 2 mating assay, a signal-isolation approach, to investigate how isolated and combined signals affect mate choice by females. Our results showed that both signals in isolation or in combination result in similar female responses (i.e., mating success, latency to copulation, female attention ratio). The presence of both vibratory and UV signals affects mating frequency, with no significant differences in mating frequency when vibratory and UV signals are presented in isolation or in combination. These results support the efficacy-based backup hypothesis. We therefore conclude that vibratory and UV signals have equivalent effects in predicting mating success in C. umbratica.