Molybdenum oxide (MoOx) has been used as a suitable interfacial modifier for emergent solar cells. However, the relatively low electrical conductivity of such material requires a thickness less than 10 nm of MoOx, which may lead to an uncovered surface and reduce the effect of interfacial modification. In this work, MoO3 and Au powder were co-evaporated and MoOx:Au composite coatings with a thickness of 10-20 nm were obtained. They are highly transparent, more compact and conductive, and contain a higher percentage of MoO2 compound than MoOx films without Au. The composite films were co-evaporated on top of the hole transport layer (Spiro-MMeOTAD) in perovskite solar cells (PSCs). The insertion of a MoOx:Au composite film, with a thickness of 14.3 nm and an atomic concentration of 34% of Au, improves photovoltaic performance and stability of PSCs after 1000 h of storage in ambient conditions. It is concluded that a relatively thick MoOx:Au layer can work as an interfacial modifier to reduce the humidity and oxygen diffusion into perovskite layer and, at the same time, improve the charge transport in PSCs.