Si is widely known as a high-capacity alloying anode material for lithium-ion batteries. However, Si is electrochemically inactive in potassium-ion batteries that attract considerable interest as cost-effective alternatives to lithium-ion batteries. Herein, we report that, unlike crystalline Si showing an inert nature toward K ions, amorphous Si is available as an alloying anode material for potassium-ion batteries. Amorphous Si can store 1.1 K ions per Si atom while offering a high capacity of 1049 mA h g−1. The K ions in amorphous Si can diffuse very rapidly. Their diffusivity is almost two orders of magnitude higher than that for Na ions in amorphous Si and is even three times higher than that for Li ions in amorphous Si. The fast K ion transport in amorphous Si is ascribed to the weak electrostatic KSi attraction, fairly high carrier ion concentration, and the formation of isolated Si n (n = 3–5) clusters during the potassiation process. This work suggests that, despite alloying with large K ions, amorphous Si anodes can show high performance in both capacity and rate capability.