MoS2 nanoribbons have attracted increased interest due to their properties, which can be tailored by tuning their dimensions. Herein, the growth of MoS2 nanoribbons and triangular crystals formed by the reaction between films of MoOx (2<x<3) grown by pulsed laser deposition and NaF in a sulfur-rich environment is demonstrated. The nanoribbons can reach up to 10 µm in length, and feature single-layer edges, forming a monolayer–multilayer junction enabled by the lateral modulation in thickness. The single-layer edges show a pronounced second harmonic generation due to the symmetry breaking, in contrast to the centrosymmetric multilayer structure, which is unsusceptible to the second-order nonlinear process. A splitting of the Raman spectra is observed in MoS2 nanoribbons arising from distinct contributions from the single–layer edges and multilayer core. Nanoscale imaging reveals a blue-shifted exciton emission of the monolayer edge compared to the isolated MoS2 monolayers due to built-in local strain and disorder. We further report on an ultrasensitive photodetector made of a single MoS2 nanoribbon with a responsivity of 8.72 × 102 A W−1 at 532 nm, among the highest reported up-to-date for single-nanoribbon photodetectors. These findings can inspire the design of MoS2 semiconductors with tunable geometries for efficient optoelectronic devices.