I'm afraid I disagree with the other answers listed here. While it is true that when one were to "cry, 'foul'", in the act of doing so the phrase could conceivably be argued grammatically to be an interjection, that notwithstanding in the context of the original question it refers to the concept of one crying, "Foul play!" (which, AFAIK, is the original context in which the phrase was introduced into common vernacular). Strangely, I was unable to corroborate the etymological origins of the phrase in a brief search, but Google's Ngram Viewer suggests the phrase entered use circa 1820, 8 years prior to the introduction of the "foul ball" in baseball. "Foul play", on the other hand, has been used since at least 1636.
All of that aside, and regardless of the (debatable) interjectory nature of one crying, "Foul play!" the word "foul" is still an adjective describing the nature of the observed shenanigans. Moreover, I'd argue that "Foul play!" is not strictly an interjection (such as "Yikes!" or "Wow!") regardless of the exclamation point at the end of the sentence. An interjection conveys emotion and nothing else, whereas the statement in question conveys, "[I believe] foul play [is taking place]!" and the extraneous exposition is simply implicit.
TL;DR: the word "foul" in the context of the original statement is an adjective, defining the nature of the play being observed. The accepted answer above, suggesting that it is a noun, is certainly incorrect by any measure.