On Saturday, May 2 at around 1pm, Kyra Aish, eight, Mali Aish, 10, and Lucia Aish, six, were using their fishing nets to look for crabs and oysters in the River Adur.
At around 1pm, they called out to their parents that they had found a seahorse - and their disbelief was unfounded when they saw the creature in the net.
They found the fish at low tide, north of Norfolk Bridge and east of the scout hut.
It was admired before they returned it to the river to stop seagulls taking it.
Their grandmother Susan Gale shared their discovery online. She said: "I posted this on the New Shoreham Facebook page and was amazed at the interest it has received with comments and 500 likes."
She contacted the Wildlife Trusts and The Sea Horse Trust, and got a reply from the latter that the seahorse was a female Short Snouted Seahorse, and would add the sighting to their database.
They said: "Seahorses are commonly found in estuaries and they have even been found just going into freshwater, which we presume is to chase food types (mainly small crustacea). Due to the arrangement of their gills they can cope with freshwater for short periods of time.
"Our two native Seahorses the Spiny (Hippocampus guttulatus) and the Short Snouted (Hippocampus hippocampus) are found fully around the British Isles with the most northerly record in the Shetland Isles. Both are found in equal numbers in the English Channel but have differing depths and habitats as preference determines at certain times of the year."