Two-dimensional (2D) molybdenum trioxide (MoO₃) with mono- or few-layer thickness can potentially advance many applications, ranging from optoelectronics, catalysis, sensors, and batteries to electrochromic devices. Such ultrathin MoO₃ sheets can also be integrated with other 2D materials (e.g., as dopants) to realize new or improved electronic devices. However, there is lack of a rapid and scalable method to controllably grow mono- or few-layer MoO₃. Here, we report the first demonstration of using a rapid (<2 min) flame synthesis method to deposit mono- and few-layer MoO₃ sheets (several microns in lateral dimension) on a wide variety of layered materials, including mica, MoS₂, graphene, and WSe₂, based on van der Waals epitaxy. The flame-grown ultrathin MoO₃ sheet functions as an efficient hole doping layer for WSe₂, enabling WSe₂ to reach the lowest sheet and contact resistance reported to date among all the p-type 2D materials (∼6.5 kΩ/□ and ∼0.8 kΩ·μm, respectively). These results demonstrate that flame synthesis is a rapid and scalable pathway to growing atomically thin 2D metal oxides, opening up new opportunities for advancing 2D electronics.