Yes. Hippos are vertebrates. The common hippopotamus (Hippopotamus amphibius) is placed in phylum Chordata and subphylum Vertebrata, the group of animals with a backbone.
ITIS, the Integrated Taxonomic Information System maintained with U.S. federal partners, lists the species under Vertebrata and class Mammalia (order Artiodactyla, family Hippopotamidae). That means hippos are mammals with a vertebral column, not invertebrates.
Source: ITIS — Hippopotamus amphibius