Effects of different reaction parameters in the hydrothermal synthesis of molybdenum oxides (MoO₃) were investigated and monoclinic (β-) MoO₃ was prepared hydrothermally for the first time. Various temperatures (90/210 °C, and as a novelty 240 °C) and durations (3/6 h) were used. At 240 °C, cetyltrimethylammonium bromide (CTAB) and CrCl₃ additives were also tested. Both the reaction temperatures and durations played a significant role in the formation of the products. At 90 °C, h-MoO₃ was obtained, while at 240 °C the orthorhombic (α-) MoO₃ formed with hexagonal rod-like and nanofibrous morphology, respectively. The phase transformation between these two phases was observed at 210 °C. At this temperature, the 3 h reaction time resulted in the mixture of h- and α-MoO₃, but 6 h led to pure α-MoO₃. With CTAB the product was bare o-MoO₃, however, when CrCl₃ was applied, pure metastable m-MoO₃ formed with the well-crystallized nanosheet morphology. The gas sensing of the MoO₃ polymorphs was tested to H₂, which was the first such gas sensing study in the case of m-WO₃. Monoclinic MoO₃ was found to be more sensitive in H₂ sensing than o-MoO₃. This initial gas sensing study indicates that m-MoO₃ has promising gas sensing properties and this MoO₃ polymorph is promising to be studied in detail in the future.