An insulation structure for a splice portion, a harness mounting plate, and a wiring unit that are capable of reducing labor-hours that are required when insulating a splice portion where a plurality of electrical wires are connected by a splice terminal. A plurality of electrical wires are connected by a splice terminal, wherein an opening portion that allows the splice portion to be inserted thereinto and removed therefrom is formed in a splice housing portion that surrounds and insulates the splice portion, and the insulation structure is provided with a separation restricting portion that restricts the splice portion that is housed in the splice housing portion from becoming separated from the opening portion.