The Honolulu Star-Advertiser reported Thursday that Cameron Cayaban pleaded guilty in federal court to harassing, harming or pursuing an endangered species.
A federal magistrate judged imposed $100 in fines and fees.
Cayaban was charged with slapping a Hawaiian monk seal at Kalaeloa's White Plains Beach in March.
His lawyer says Cayaban was overcome when he saw the seal, ran up to the endangered animal and touched it.
Witnesses reported it to military police.