Reformism is a political doctrine advocating the reform of an existing system or institution instead of its abolition and replacement.

Within the socialist movement, reformism is the view that gradual changes through existing institutions can eventually lead to fundamental changes in a society's political and economic systems. Reformism as a political tendency and hypothesis of social change grew out of opposition to revolutionary socialism, which contends that revolutionary upheaval is a necessary precondition for the structural changes necessary to transform a capitalist system to a qualitatively different socialist system. Responding to a pejorative conception of reformism as non-transformational, non-reformist reform was conceived to prioritize human needs over capitalist needs.

As a doctrine, centre-left reformism is distinguished from centre-right or pragmatic reform, which instead aims to safeguard and permeate the status quo by preventing fundamental structural changes to it whereas leftist reformism posits that an accumulation of reforms can eventually lead to the emergence of entirely different economic and political systems than those of present-day capitalism and bureaucracy.