Details of the transition from a sinistral, paucispiral larval shell to the unique bivalved shell of the adult sacoglossan sea slug Berthelinia singaporensis Jensen, 2015 (Mollusca: Gastropoda: Heterobranchia) are described for the first time using digital microphotography, video-recording, and SEM. The transition lasts only a few hours on day 4 post-hatching, probably explaining why the details were overlooked in previous studies of other species of Berthelinia. The formation of a visor-like shell, which marks the beginning of metamorphosis, is initiated during the first three days after hatching, during which the velar lobes are reduced. Shell growth initiates from the inside of the larval shell after the mantle fold has detached from the aperture. There is a narrow gap between the aperture of the larval shell, which will be called the protoconch in succeeding stages, and the new shell growth. Next, a short, narrow, somewhat flared fold is formed at the right side edge of the visor. This allows further shell growth to be directed downwards on the right side. The shell is soft and flexible, and alternately flares to form a ring-like skirt surrounding the protoconch aperture, and bends downwards to form a laterally compressed shell. A fracture line is visible on the inside of the shell to the right of the fold in the visor and shortly after, the future hinge line is also visible externally. The operculum, here illustrated by SEM for the first time in a sacoglossan, is cast off at this time, marking the completion of shell metamorphosis. After this, the shell grows anteriorly as well as posteriorly, the adductor muscle becomes more prominent, and the neck region as well as the tail elongate. Colouration appears gradually, first as brown oily spots in the mantle and visceral mass, later as uniformly green in the entire body. Spawning and early, intra-capsular development have also been observed and documented. This follows the basic pattern of other species of Berthelinia and of other sacoglossans with lecithotrophic development. The spawning specimens used in this study had shell lengths of 4–5 mm. Egg masses are flat and band-shaped, of variable lengths, usually approximately 1 mm wide. They contained 24–164 eggs. Egg diameter is 85 µm, capsule size ≈ 330 × 250 µm. Veliger larvae with eyes, statocysts, an operculum, and a distinct propodium hatch after ten days. Larval shells at hatching are ca. 250 µm in maximum diameter.