Moral Skepticism is the meta-ethical theory that no-one has any moral knowledge (or the stronger claim that no-one can have any moral knowledge). It holds that we are never justified in believing that, and never know whether, moral claims are true. This is not the same as arguing that all moral claims are false, which is the position of Moral Nihilism or Error Theory. It is related to, and takes its general reasoning from, Pyrrhonian or Academic Skepticism traditions in Epistemology.

Some commentators would argue that the label Moral Skepticism includes positions such as Moral Nihilism and Non-Cognitivism on the grounds that these theories also entail doubts about the validity of moral claims, but they are often considered quite separately. All of these positions, however, are particularly opposed to any type of Moral Realism.

Some see it as absurd and an abdication of intellectual responsibility to hold that any moral theory can be refuted merely by showing that it leads to Moral Skepticism. Skeptics who deny that we have reason to believe or obey ethical claims like "slavery is morally wrong" (or "terrorism" or "child abuse") are seen as misguided and dangerous, contrary to common sense, and likely to lead to immorality.

Perhaps the most famous Moral Skepticist was Friedrich Nietzsche, and more recent proponents include the Error Theorist J. L. Mackie (1917 - 1981), the philosopher of science Michael Ruse (1940 - ) and Richard Joyce.

Different versions of Moral Skepticism deny or doubt moral knowledge, justified moral belief, moral truth, moral facts or properties, and reasons to be moral:

• Pyrrhonian Moral Skepticism is content to merely doubt that moral knowledge is even possible.
• Dogmatic Moral Skepticism claims that no-one ever knows that any substantive moral belief is true, nor can be justified in holding any such beliefs.
• Practical Moral Skepticism denies that there is always enough reason for moral action (as opposed to the more dogmatic denial that there is ever adequate reason).