The present disclosure provides a liquid treatment device, a liquid treatment method, and a plasma treatment liquid each capable of efficiently generating plasma and treating a liquid in a short time period. A liquid treatment device according to the present disclosure includes a first electrode, a second electrode disposed in a liquid to be treated, an insulator disposed around the first electrode with a space between the first electrode and the insulator, the insulator has an opening portion in a position in contact with the liquid to be treated, a power supply that applies voltage between the first electrode and the second electrode, and a supply device supplying a liquid to the space before the power source applies the voltage.