The first report on the occurrence of the remarkable and highly ephemeral sap-sucking sea slugs Elysia bangtawaensis and E. leucolegnote from the newly grown mangrove forest in the Prince of Songkla University, Pattani, Thailand. Elysia was surveyed by exploring from the inner part to the floor front zone of the mangrove area. The various sizes and numbers of E. bangtawaensis were clumped distribution in some microhabitats throughout the area, while E. leucolegnote was distributed in the floor front zone of the area. Both species have similar external morphological characters with conspecifics previously reported in Pattani Bay, Gulf of Thailand, Andaman Sea, and elsewhere. E. bangtawaensis showed a surprisingly larger size than previously reported. This discovery is important in identifying the changes in ecosystems within the area to support the diversity of organisms that will come to use the area in the future.