Two-dimensional molybdenum disulfide (MoS2) has recently drawn major attention due to its promising applications in electronics and optoelectronics. Chemical vapor deposition (CVD) is a scalable method to produce large-area MoS2 monolayers, yet it is challenging to achieve shape-uniform, high-quality monolayered MoS2 grains as random, diverse crystallographic orientations and various shapes are produced in the same CVD process. Here, we report the growth of high-quality MoS2 monolayers with uniform triangular shapes dominating (up to 89%) over other shapes on both SiO2/Si and sapphire substrates. The new confined-space CVD process prevents contamination and helps regulate the Mo/S ratio during the deposition. The as-grown triangular MoS2 monolayers exhibit grain sizes up to 150 μm and possess better crystalline properties and lighter n-type doping concentration than those of the monolayers grown by common CVD methods. The corresponding field effect transistor devices show high electron mobilities of 50–60 cm2 V–1 s–1 and positive threshold voltages of 21–35 V. This mild n-type behavior makes it possible to regulate the formation of excitons by back-gate voltage due to the interaction of excitons with free charge carriers in the MoS2 channel. As a result, gate-tunable photoluminescence (PL) effect, which is rarely achievable for MoS2 samples prepared by common CVD or mechanical exfoliation, is demonstrated. This study provides a simple versatile approach to fabricating monolayered crystals of MoS2 and other high-quality transition metal dichalcogenides and could lead to new optoelectronic devices based on gate-tunable PL effect.