A Private Member's Bill is a proposed law introduced by a backbencher, whether from the government or the opposition side, to a legislature or parliament. In most parliaments within the Westminster System of parliamentary democracy, the overwhelming majority of Bills introduced are proposed by members of the Government. However some parliamentary time is often set aside so that backbenchers may introduce Bills also.

In reality, few backbench Private Member's Bills make it to enactment. In some cases, controversial measures that a government does not want to take responsibility for may be introduced by backbenchers, with the government secretly backing the measure and ensuring its passage. The Abortion Act, 1967 was enacted in the United Kingdom though this means. The Act itself was introduced by a Liberal Party Member of Parliament, David Steel, but was enacted through the support that came from Harold Wilson's Home Secretary, Roy Jenkins.

The United Kingdom parliament has a long history of enacting Private Member's Bills. In contrast Oireachtas Éireann (Ireland's two chamber parliament) rarely passes Private Members Bills, with the overwhelming number of Bills being passed all being introduced by members of the Irish government.