肯定后件（affirming the consequent），是一种形式谬误，其形式如下：

若P则Q

Q

因此，P

范例：

如果有火灾，就会有浓烟。

有浓烟。

因此，有火灾。

Affirming the consequent, sometimes called converse error, fallacy of the converse or confusion of necessity and sufficiency, is a formal fallacy of inferring the converse from the original statement. The corresponding argument has the general form:

An argument of this form is invalid, i.e., the conclusion can be false even when statements 1 and 2 are true. Since P was never asserted as the only sufficient condition for Q, other factors could account for Q (while P was false).