Why would it not be? An epic poem is a long narrative poem, and it generally depicts heroic tales.
Beowulf meets both of those requirements and is generally considered a classic epic poem, just like the Iliad and the Odyssey are (not to mention Sir Gawain and the Green Knight).
I suppose we could define "epic" more narrowly, so it excludes any poetry that doesn't meet certain parameters. For Homeric epic and most Western epic to follow, for example, the poem begins with an "invocation to the Muse," which basically means that the poet marks the beginning of an epic by calling upon the mythical Muse of poetry and music to help him (it's always a "him") write the poem. Homer began the Iliad with a line that reads something like: "Sing, O goddess, the anger of Achilles son of Peleus!" and Milton (in Paradise Lost) began with "Sing Heav'nly Muse, that on the secret top / Of Oreb, or of Sinai, didst inspire / That Shepherd, who first taught the chosen Seed...." There is, however, no reason to disregard an epic from another place and time simply because it doesn't fit our more stringent "requirements" for epic poetry.