Leptolinae (or Leptolina) are a subclass of the Hydrozoa Class (biology) in the cnidarian phylum. They contain the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the Hydrozoa in older classifications and were placed at order rank. They also include, however, the highly advanced colonial jellies of the Siphonophora, which were not included in the "Hydroida".

Leptolinae (or Leptolina) are a subclass of the Hydrozoa Class (biology) in the cnidarian phylum. They contain the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the Hydrozoa in older classifications and were placed at order rank. They also include, however, the highly advanced colonial jellies of the Siphonophora, which were not included in the "Hydroida".