議會制又稱內閣制、議會民主制（英語：Parliamentary system），是一種政治制度，特點是「議會至上」，政府首脑（總理或首相）權力來自議會，授權有兩種途徑：第一是議會改選後的多數議席支持，第二是政府首腦贏得議會的信任投票。因此，所屬政党未能贏得議會大選的政府首腦連同其內閣必須提出辭職，而未能通過議會信任投票的政府首腦，連同其內閣也必須辭職，由議會席位居多數的黨派中協商選舉產生新的首腦與內閣。議會制下的政府融合行政權與立法權（Fusion of Powers），確保其所領導的政府具民意基礎。
在採用這種政治體制的國家中，立法机构與行政機關並不完全分立。议会是国家权力的中心。議會民主制一般實行於民主國家，因此存在著多個參加議會選舉的政黨。議會內閣制的政府首脑（行政首長）與國家元首（head of state）分開，其國家元首通常是象徵性元首，儀式性職務，不享有實際的行政權，其权力一般仅限于任命议会中的多数党领袖或者多党政治联盟领袖担任政府总理。少數議會制國家有民選總統的產生（如愛爾蘭、冰島），總統有部分職責如任命總理（需議會同意）與處理國防與外交事務。
^In Bangladesh, a caretaker government during parliamentary elections. The Caretaker government is headed by a Chief Adviser and a group of neutral, non-partisan advisers chosen from the civil society. During this time, the president has jurisdiction over the defence and foreign affairs ministries.
^Collective presidency consisting of three members; one for each major ethnic group.
^Formerly a semi-presidential republic, it is now a parliamentary republic according to David Arter, First Chair of Politics at Aberdeen University, who in his "Scandinavian Politics Today" (Manchester University Press, revised 2008 ISBN9780719078538), he quotes Nousiainen, Jaakko. From semi-presidentialism to parliamentary government: political and constitutional developments in Finland. Scandinavian Political Studies. June 2001, 24 (2): 95–109. doi:10.1111/1467-9477.00048. as follows: "There are hardly any grounds for the epithet 'semi-presidential'." Arter's own conclusions are only slightly more nuanced: "The adoption of a new constitution on 1 March 2000 meant that Finland was no longer a case of semi-presidential government other than in the minimalist sense of a situation where a popularly elected fixed-term president exists alongside a prime minister and cabinet who are responsible to parliament (Elgie 2004: 317)". According to the Finnish Constitution, the President has no possibility to rule the government without the ministerial approval, and substantially has not the power to disband the parliament under its own desire. Finland is actually represented by its Prime Minister, and not by its President, in the Council of the Heads of State and Government of the European Union. The 2012 constitutional amendments reduced the powers of the President even further.
^ 5.05.15.2Combines aspects of a presidential system with those of a parliamentary system. The president is elected by parliament and holds a parliamentary seat, much like a prime minister, but is immune from a vote of no confidence (but not their cabinet), unlike a prime minister.
^ 6.06.16.2Combines aspects of a presidential system with those of a parliamentary system. The president is elected by parliament but does not hold a parliamentary seat, and is immune from a vote of no confidence (as well is their cabinet), unlike a prime minister.
In many cases, the Governor-General or monarch has a lot more theoretical, or constitutional, powers than they actually exercise, except on the advice of elected officials, per constitutional convention. For example, the Constitution of Australia makes the GG the head of the executive branch (including commander-in-chief of the armed forces), although they seldom ever use this power, except on the advice of elected officials, especially the PM, which makes the PM the de facto head of government.