MoO3 has potential advantages in price and weight over more widely studied WO3 for electrochromic devices, but it typically suffers from inferior electrochromic performance. Here, it is demonstrated that 2D MoO3 nanosheets can be formed by intercalation and exfoliation of MoS2 to form partially oxidized MoSxOy, followed by complete oxidation using a solution-phase nitric acid treatment. These fully oxidized MoO3 nanosheets retain the 2D nature of exfoliated MoS2 (70% of nanosheets are less than 9 nm thick), allowing for easier ionic diffusion through the film compared to bulk MoO3 powder. When utilized as an electrochromic material, the device showed outstanding contrast (48.6% at 700 nm) and fast response time (11.3 s for bleaching, 9.8 s for coloration), which is far superior to that for standard powder MoO3 and comparable to that for many nanostructures of WO3. This shows the outstanding potential for these MoO3 nanosheets in electrochromic applications.