Limnonectes sarawakensis can be encountered in lowland and submontane rainforests of central and northern Sarawak, at elevations between 60 and 1,000 meters above sea level. Type locality are the lowlands of Lambir Hills National Park (Sabah). The species occurs together with L. gunungensis on the High Plateau on Gunung Dulit, Sarawak. At Gunung Mulu national Park, Sarawak, it occus with L. phylax in the lowland forests of Gunung Mulu National Park, Sarawak, yet seemingly in different habitats (alluvial forest in L. sarawakensis vs dipterocarp/karst forest in L. phylax).

Males measure approx. 25–30 mm in snout-vent length and adult females 30–34 mm. 

Limnonectes sarawakensis displays the typical traits of of the palavanensis-species group, such as a small body size (snout–vent length < 40 mm), low and inconspicuous odontoid processes on the lower jaw in both males and females, and a finely shagreened dorsal surface with prominent longitudinal dorsolateral glandular ridges. The interorbital distance is equal to or greater than the width of the upper eyelid. The toes are not fully webbed, and a characteristic horseshoe-shaped dermal ridge is present in the middle of the dorsum, just behind the point where the forelimbs insert.

This species is distinguished from the other members of the species group by a combination of features: It has extensive toe webbing and a strongly curved supratympanic fold. The pineal spot is weakly discernible and located anterior to the front edge of the upper eyelids. The vomerine processes are long and separated from both the choana and from each other by about half the length of each individual process. The postpalpebral ridge is well developed, extending from the upper eyelid to the front of the pelvis. The median ridge is indistinct from the level of the rear edge of the upper eyelid to the horseshoe-shaped ridge but becomes more pronounced between that ridge and the vent. The lumbar ridge is well developed but interrupted.

The advertisement call is a long, rising trill composed of 26 to 32 notes, lasting up to 2.0 seconds.