A safe seat is an electoral district which is regarded as fully secure, for either a certain political party, or the incumbent representative personally or a combination of both. With such seats, there is very little chance of a seat changing hands because of the political leanings of the electorate in the constituency concerned or the popularity of the incumbent member. This contrasts with a marginal seat in which a defeat for the seat holder is considered possible. In systems where candidates must first win the party's primary election or preselection, the phrase "tantamount to election" is often used to describe winning the dominant party's nomination for a safe seat.

There is a spectrum between safe and marginal seats. Supposedly safe seats can still change hands in a landslide election, such as Enfield Southgate being lost by the Conservatives (and then-potential future party leader Michael Portillo) to Labour at the 1997 UK general election, whilst other seats may remain marginal despite large national swings, such as Gedling, which Labour narrowly won in every election for twenty years until the 2019 general election, despite having both major victories and defeats during this time. Gedling would still be seen as a marginal seat, even though it had been held by Labour for a long time. Safe seats are usually seats that have been held by one party for a long time, but the two concepts are not interchangeable.

In countries with parliamentary government, parties often try to ensure that their most talented or influential politicians are selected to contest these seats – in part to ensure that these politicians can stay in parliament, regardless of the specific election result, and that they can concentrate on ministerial roles without needing to spend too much effort on managing electorate-specific issues.

Candidate selection for a party's safe seats is usually keenly contested, although many parties restrict or forbid challenges to the nomination of sitting members. The selection process can see the incumbent party, untroubled by the need to have a representative that must appeal to a broader electorate, take the opportunity to choose a candidate from the more ideological reaches of the membership. Opposing parties will often be compelled to nominate much less well-known individuals (such as backroom workers or youth activists in the party), who will sometimes do little more than serve as paper candidates who do little or no campaigning, or will use the contest to gain experience so that they become more likely to be selected for a more winnable seat. In some cases (especially in the United States), these seats may go uncontested by other major parties.

Safe seats can become marginal seats (and vice versa) gradually as voter allegiances shift over time. This shift can happen more rapidly for a variety of reasons. The retirement or death of a popular sitting member may make a seat more competitive, as the accrued personal vote of a long-serving parliamentarian will sometimes have resisted countervailing demographic trends. An independent or third-party candidate with an ideology close to that of the incumbent party may also be able to make a more credible challenge than more established parties, but these factors can combine: a retiring third-party member may turn a safe seat for that party into a marginal seat. For instance, in Berwick-upon-Tweed, with the retirement of the popular incumbent Alan Beith, the seat was no longer safe for the Liberal Democrats.

Traditionally safe seats can also be more vulnerable in by-elections, especially for governing parties. Safe seats may also become marginal if the sitting member is involved in scandal: in 1997, Tatton was gained from the Conservatives by an anti-sleaze independent candidate, despite the majority previously being that of a very safe seat for the Conservatives. The incumbent, Neil Hamilton, had been mired in controversy, and was defeated by the veteran BBC journalist Martin Bell, who was aided by the decision of the main opposition parties (Labour and the Liberal Democrats) not to field candidates. Without such pacts, a split vote is more likely under a first past the post electoral system, as in the UK.

Opposition supporters in safe seats have restricted means to affect election outcomes, and thus the incumbent parties can, in theory, decide to ignore those supporters' concerns, as they have no direct effect on the election result. Even those voters who are moderate supporters of the incumbent party may be disenfranchised by having a representative whose views may be more extreme than their own. Political objectors in such areas may experience marginalisation from wider democratic processes and political apathy. This is often regarded as undemocratic, and is a major argument in favour of various multi-member proportional representation election methods. Safe seats may receive far less political funding than marginal seats, as the parties will attempt to "buy" marginal seats with funding (a process known in North America and Australia as "pork barrelling"), while ignoring safe seats which will reliably fall to the same party every time; this is especially true in cases where the safe seat is held by the minority party.

In countries that do not apply the first past the post system, many of which equally operate a geographic division-based system, selected or party sub-nominated candidates can be allocated a safer or more tenuous list position. If a party is strong enough nationwide to gather representations in all subdivisions, the top candidate(s) on each list tend to be very safely elected to parliament. This is seen in the extremely proportional election systems of the Nordic countries, for example. Safe seats and candidates can be avoided altogether by a purposefully marginal-preference allocation of all divisions, ensuring all divisions are near-identically demographically diverse which may be achieved by pairing non-adjoining areas.