Tetrastemma flavidum is not a well-characterized species and the records from the British Isles are confused.
It reaches a length of 14-15 mm but is generally only 0.5-0.75 mm wide. The body is rather flattened, bluntly rounded at its posterior tip and either rounded or slightly tapered at the head. The four eyes are very small but usually distinct (T. flavidum). In colour typically a bright pink, smaller individuals may be tinged pale yellowish or reddish and appear more or less transparent with their various internal organs clearly visible in transmitted light. The lateral margins are translucent. Mature individuals are found during the summer months.
The species is found in mud, fine sand or gravel, or among the branches of laminarian holdfasts, from the lower shore to depths of up to 100 m. Specimens found infrequently in the branchial cavity of ascidians such as Ascidia mentula may represent a different species or merely be in the 'host' by chance; there is no evidence at present to suggest that the species possesses even occasional commensal or parasitic habits.
The geographic range extends from the British Isles and Scandinavia to the Mediterranean and Red Sea.