Seahorses reproduce sexually through internal fertilization and spawn during every season. With Hippocampus erectus, as with all seahorses, it is the male that cares for the young. Male seahorses have an incubation pocket, similar to the pouch of a marsupial mammal, on the lower side of the tail, with an opening that can be closed off. During courtship, female Hippocampus erectus spray between 250 and 650 eggs into the male brood pocket, depending on the size of the individual. Development in the brood pocket lasts between 20 and 21 days. After hatching, the free embryos are carried in the pouch until they are capable of fairly active swimming. When it is time for the incubated young to be born, the prospective father holds fast to a plant stem or some other object by his prehensile tail. He bends rapidly, sharply, backward and forward; the pouch opens and a baby seahorse pops out. With brief intervals between births, the jerking motions are repeated until the pouch is emptied. Each infant seahorse emerges head first and is a swimming, independent miniature of the adult. Newborn Hippocampus erectus are about 5/8 of an inch in length, and within 8-10 months reach their maximum size of 5 inches in length. (Moyle and Cech, 1982, pg. 120; Schultz, 1948, pg. 114; National University of Singapore)

One of the complications of seahorse birth is that some of the unhatched seahorses may die within the pouch before birth, and this soon results in the formation of gas. The male then virtually becomes a balloon and is quickly buoyed to the surface, where he is immediately be picked off by some hungry fish. (Herald, 1961, pg. 148)