Weak atheism or negative atheism is the lack of belief in any God or gods, without a positive denial of the existence of any god or gods. Weak atheism contrasts with strong atheism, which asserts that gods do not exist, and theism, which asserts that there is at least one god. The weak atheist generally gives a broad definition of atheism as "lack or absence of belief in god or gods", which defines atheism as a range of positions that entail non-belief, disbelief, doubt, or denial of theism. A narrower definition of atheism as "denial of the existence of gods" is also in common use. Those who favor this definition prefer to use other terms, such as agnosticism, for skepticism of or non-belief in theism.

Some weak atheists may simply hold no opinion on the existence or non-existence of God or gods. Others may find arguments on both sides of the question equally compelling. Yet others may doubt or disbelieve the existence of God or gods, being unconvinced by the evidence or proofs put forward by theists, but hold that it is not currently known whether or not gods exist.

The position that it is not known whether God or gods exists is called weak agnosticism. A stronger position is strong agnosticism, the view that it is not possible ever to know whether God or gods exist or not. Agnosticism, in both strong and weak forms, is not necessarily an atheist position; it is a position regarding what is known, and can be known, about the existence of God or gods. A theist can also take the position that he does not know (and if a strong agnostic, cannot know) whether God or gods exist, although he believes it, through faith.

Weak atheists often argue that their position is the default one; that it is not known whether or not gods exist (and if the strong agnostics are right, can never be known); that neither theists nor strong atheists fulfill their burden of proof; and that theists and strong atheists therefore rely on faith. For these weak atheists, beliefs which cannot be fully justified and which rest on faith, are untenable, making both theism and atheism untenable.

For more on the debate between strong and weak atheism, consult the article on strong atheism. See also the articles on agnosticism, weak agnosticism, strong agnosticism.