A private member's bill is a bill (proposed law) introduced into a legislature by a legislator who is not acting on behalf of the executive branch. The designation "private member's bill" is used in most Westminster system jurisdictions, in which a "private member" is any member of parliament (MP) who is not a member of the cabinet (executive). Other labels may be used for the concept in other parliamentary systems; for example, the label member's bill is used in the Scottish Parliament and the New Zealand Parliament, the term private senator's bill is used in the Australian Senate, and the term public bill is used in the Senate of Canada. In legislatures where the executive does not have the right of initiative, such as the United States Congress, the concept does not arise since bills are always introduced by legislators (or sometimes by popular initiative).

In the Westminster system, most bills are "government bills" introduced by the executive, with private members' bills the exception; however, some time is set aside in the schedule for reading such bills. They may be introduced by non-ministerial MPs from government-supporting parties (backbenchers), by members of opposition parties (frontbencher or backbencher), or by independents or crossbenchers. The Israeli Knesset has a long history of enacting private members' bills: a slight majority of the laws passed by it originated as private members' bills, and thousands more are introduced without being passed. In contrast, the Oireachtas (parliament) of the Republic of Ireland rarely passes private members' bills, with the overwhelming number of bills being passed being introduced by members of the cabinet.

A private member's bill is not to be confused with a private bill, which is a bill that only affects an individual citizen or group.