Robert Frost's first published poem was "My Butterfly: An Elegy". It was published in the New York Independent on September 9, 1894, when Frost was just 20 years old.
However, it's worth noting that this poem was written under the pseudonym "Launcelot". Frost didn't use his own name until his second published poem, "The Trial by Existence," which appeared in the same magazine later that year.