To date, the synthesis of crystalline ZnO nanostructures was often performed under high temperatures and/or high pressures with tiny output, which limits their commercial applications. Herein, we report the progress on synthesizing single-crystalline ZnO nanosheets under ambient conditions (i.e., room temperature (RT) and atmospheric pressure) based on a sonochemistry strategy. Furthermore, their controllable growth is accomplished by adjusting the pH values of solutions, enabling the tailored crystal growth habits on the polar-charged faces of ZnO along c-axis. As a proof of concept for their potential applications, the ZnO nanosheets exhibit highly efficient performance for sensing ammonia at RT, with ultrahigh sensitivity (S = 610 at 100 ppm), excellent selectivity, rapid detection (response time/recover time = 70 s/4 s), and outstanding detection limit down to 0.5 ppm, superior to those of all pure ZnO nanostructures and most ZnO-based composite counterparts ever reported. The present work might open a door for controllable production of ZnO nanostructures under mild conditions, and facilitate the exploration of modern gas sensors for detecting gaseous molecules at RT, which underscores their potential toward practical applications in opto-electronic nanodevices.