Controllable nucleation sites play a key role in the selective growth of heterostructures. Here, we are the first to report a one-pot strategy to realize the confined and selective growth of large MoS2/WS2 lateral and vertical heterostructures. A hydroxide-assisted process is introduced to control the nucleation sites, thereby realizing the optional formation of lateral and vertical heterostructures. Time-of-flight secondary ion mass spectrometry verifies the critical role of hydroxide groups toward the controllable growth of these heterostructures. The size of the as-grown MoS2/WS2 lateral heterostructures can be as large as 1 mm, which is the largest lateral size reported thus far. The obtained MoS2/WS2 heterostructures have a high carrier mobility of ∼58 cm2 V-1 s-1, and the maximum on/off current ratio is >108. This approach provides not only a pathway for the selective growth of large MoS2/WS2 lateral and vertical heterostructures but also a fundamental understanding of surface chemistry for controlling the selective growth of transition-metal dichalcogenide heterostructures.