We report the growth of large-area monolayer MoS_2(1−x)Se_2x alloys with controlled morphologies using a low-pressure chemical vapor deposition (CVD) method. MoS_2(1−x)Se_2x alloys with different morphologies, created using the same growth time, have been observed by controlling the gaseous MoO₃ precursor on substrates placed in regions with different temperatures. TEM observations clearly reveal that the as-synthesized monolayer MoS_2(1−x)Se_2x alloy is crystalline, with a hexagonal structure. XPS, Raman mapping, and EDS mapping clearly show the homogeneous substitution of ∼2 atomic weight % Se through the whole crystal. Compared with a pristine CVD-grown monolayer of MoS₂, the optical band gap differs by 4.52%, from 1.77 eV to 1.69 eV. Additionally, back-gated transistors fabricated on the monolayer MoS_2(1−x)Se_2x alloy exhibit n-type behavior at a current on/off ratio of ∼10⁴ and a high mobility value of 8.4 cm² V⁻¹ s⁻¹.