A working hypothesis is a hypothesis that is provisionally accepted as a basis for further ongoing research in the hope that a tenable theory will be produced, even if the hypothesis ultimately fails. Like all hypotheses, a working hypothesis is constructed as a statement of expectations, which can be linked to deductive, exploratory research in empirical investigation and is often used as a conceptual framework in qualitative research. The term "working" indicates that the hypothesis is subject to change.

Use of the phrase "working hypothesis" goes back to at least the 1850s.

Charles Sanders Peirce came to hold that an explanatory hypothesis is not only justifiable as a tentative conclusion by its plausibility (by which he meant its naturalness and economy of explanation), but also justifiable as a starting point by the broader promise that the hypothesis holds for research. This idea of justifying a hypothesis as potentially fruitful (at the level of research method), not merely as plausible (at the level of logical conclusions), is essential for the idea of a working hypothesis, as later elaborated by Peirce's fellow pragmatist John Dewey.

In 1890, and again in 1897, Thomas Chrowder Chamberlin wrote "The method of multiple working hypotheses", in which he advocated the importance of simultaneously evaluating several hypotheses, rejecting those that conflict with available data, and ending with the one hypothesis supported by the data. This stood in contrast to what he called the single ruling theory, which encouraged scientists to find supporting data and not challenge it with difficult tests. The paper is considered a landmark on the scientific method, was an inspiration for the approach called strong inference, and was reprinted in 1965.

Peirce held that, as a matter of research method, an explanatory hypothesis is judged and selected for research because it offers to economize and expedite the process of inquiry, by being testable and by further factors in the economy of hypotheses: low cost, intrinsic value (instinctive naturalness and reasoned likelihood), and relations (caution, breadth, and incomplexity) among hypotheses, inquiries, etc. (as in the game of Twenty Questions). The Century Dictionary Supplement definition of "working hypothesis" reflects that perspective; Peirce may or may not have written it. Peirce seldom used the phrase "working hypothesis," but he once commented about a related kind of a hypothesis that it was "a hypothesis, which like the working hypothesis of a scientific inquiry, we may not believe to be altogether true, but which is useful in enabling us to conceive of what takes place." For Peirce the pragmatist, conceiving pragmatically of something meant conceiving of its effects in their conceivable implications as to informed practice in general including research.

John Dewey used the concept of the working hypothesis as a pivotal feature in his theory of inquiry. Contrary to the principles of verification and falsifiability, used in formal hypothesis testing found within dominant paradigms of 'normal' science, working hypotheses were conceived by Dewey as neither true nor false but "provisional, working means of advancing investigation," which lead to the discovery of other unforeseen but "relevant" facts. Dewey's development of the concept of the working hypothesis emerged from his contextualist epistemology in which absolute truth is unobtainable and replaced by "warranted assertability". Thus, Dewey noted:

The history of science also shows that when hypotheses have been taken to be finally true and hence unquestionable, they have obstructed inquiry and kept science committed to doctrines that later turned out to be invalid.

In Dewey's view, the working hypothesis is generated, not directly as a testable statement of, but instead in order to "direct inquiry into channels in which new material, factual and conceptual, is disclosed, material which is more relevant, more weighted and confirmed, more fruitful, than were the initial facts and conceptions which served as the point of departure".