In Westminster and other parliamentary systems, a backbencher is a member of parliament (MP) or a legislator who occupies no governmental office and is not a frontbench spokesperson in the Opposition, being instead simply a member of the "rank and file".

The term dates from 1855. The term derives from the fact that they sit physically behind the frontbench in the House of Commons. A backbencher may be a new parliamentary member yet to receive a high office, a senior figure dropped from government, someone who for whatever reason is not chosen to sit in the government or an opposition spokesperson (such as a shadow cabinet if one exists), or someone who prefers to be a background influence, not in the spotlight. By extension, those who are not reliable supporters of all of their party's goals and policies and have resigned or been forced to resign may be relegated to the back benches. For example, in British political events, Clive Lewis became a backbencher after resigning from Jeremy Corbyn's shadow cabinet over Brexit, and Boris Johnson became a backbencher again, after resigning as Foreign Secretary in Theresa May's cabinet, also over Brexit. May herself returned to the backbenches after her resignation from the premiership, to be succeeded by Johnson.

In most parliamentary systems, individual backbenchers have little power to affect government policy. However, they play a greater role in the work of the legislature itself; for example, sitting on parliamentary committees, where legislation is considered and parliamentary work is done in more detail than there is time for on the floor of the House. In addition, since backbenchers generally form the vast majority of government MPs – and even their totality in dualistic parliamentary systems, where Ministers cannot serve as MPs simultaneously, collectively they can sometimes exercise considerable power, especially in cases where the policies of the government are unpopular or when a governing party or coalition is internally split. Backbenchers carry considerable influence when the government majority is small; for example, Ariel Sharon's second government's plan of Israeli disengagement from the Gaza Strip was frequently obstructed by backbenchers from both junior partners in the right-wing coalition (such as the Mafdal and National Union) and dissenters in Sharon's own Likud party. Although the internal dissenters within the Likud eventually ejected Sharon and his supporters out of the party in late 2005, the Sharon-led Kadima party that resulted from the splinter remained in control of the government and went on to win the 2006 elections.

In some legislative assemblies, sitting at the back of the chamber is not necessarily associated with having a minor role. In Switzerland, senior figures sit in the back rows in order to have a better overview and be closer to the doors for discussions outside the plenary. In Germany, the party leaders sit in the front row, but there are no designated places for other senior figures. The term backbenchers ("Hinterbänkler") therefore refers to largely unknown MPs without much influence, regardless of where they sit. Originally, the importance of the front rows for the leaders had also to do with the fact that acoustics were often unsatisfactory before microphones were introduced. Prominent or iconic political figures can also play a backbench role, as the case of Aung San Suu Kyi illustrates: the leader of Myanmar's opposition to military rule was first elected MP in 2012 but proved only marginally involved in legislative business.

The term "backbencher" has also been adopted outside parliamentary systems, such as the United States Congress. While legislative branches in presidential systems do not share the firm front bench/back bench dichotomy of the Westminster system, the term has been used to denote junior legislators or legislators who are not part of party leadership within a legislative body. When Democrat Tim Ryan of Ohio challenged Nancy Pelosi of California for House Minority Leader in 2016, The Washington Post reported that he "emerged from the backbench — he literally sits on the last bench in the chamber".