Intracellular potassium homeostasis is a prerequisite for the optimal operation of plant metabolic machinery and plant's overall performance. It is controlled by K+ uptake, efflux and intracellular and long-distance relocation, mediated by a large number of K+-selective and non-selective channels and transporters located at both plasma and vacuolar membranes. All abiotic and biotic stresses result in a significant disturbance to intracellular potassium homeostasis. In this work, we discuss molecular mechanisms and messengers mediating potassium transport and homeostasis focusing on four major environmental stresses: salinity, drought, flooding and biotic factors. We argue that cytosolic K+ content may be considered as one of the ‘master switches’ enabling plant transition from the normal metabolism to ‘hibernated state’ during first hours after the stress exposure and then to a recovery phase. We show that all these stresses trigger substantial disturbance to K+ homeostasis and provoke a feedback control on K+ channels and transporters expression and post-translational regulation of their activity, optimizing K+ absorption and usage, and, at the extreme end, assisting the programmed cell death. We discuss specific modes of regulation of the activity of K+ channels and transporters by membrane voltage, intracellular Ca2+, reactive oxygen species, polyamines, phytohormones and gasotransmitters, and link this regulation with plant-adaptive responses to hostile environments.