Yes. William Blake is a known pantheist. As you know, a Pantheist is person who holds to the metaphysical argument that God is everything around us, and not a separate entity for which we need to build a specific path to encounter.
As part of the Romantic Pantheist movement, Blake is also joined by Coleridge, Wordsworth, even Virgil, and many more.
The evidence of his believes lays in several of his works, namely in The Marriage of Heaven and Hell, and The Lamb.
In The Marriage of Heaven and Hell the elements of pantheism are evident in the mention of "Energy" as a form of entity that is ever-present and forever will be. "Energy" would be a way for a panthesis to define what judeo-christians would define as "God". A Pantheist, however, does not view God as a separate being.
Another poem, The Lamb, though might seem like a Christian poem for its combination of Judeo Christian symbolism, in fact is more of a broader view of religion using common symbols to refer to that which is one and all. The fact that he used christian symbols in The Lamb was part of his Romantic perspective as a poet. In Romantic literature symbolism of all kinds is a broadly-used technique, and does not constitute labeling.
A separate aspect of Romanticism is the connection to nature and the embodiment of the "all is one" of Pantheism. In the poem Ah, Sunflower this is also evydent as well as the bucolic nostaligia so typical of Romantics:
Ah Sunflower, weary of time,
Who countest the steps of the sun;
Seeking after that sweet golden clime
Where the traveler's journey is done.