The head and body of male Pseudomugil ivantsoffi is semitransparent, often with a bluish or reddish hue. Iris of eye intensely blue. The opercle, abdomen and swim bladder region are silvery. The upper half of first dorsal and anterior half of second dorsal fins, and edge of anal fin are bright red. The dorsal and ventral contour posterior to second dorsal and anal fins edged with similar red; the upper and lower third of caudal fin also red. The pelvic fins are pinkish. The edges of some scales on abdomen with narrow black margins. Females overall semitransparent, lacking bright red shades of male; opercle, abdomen and swim bladder region silvery; some scales on anterior half of body with faint, fine black margins; fins mainly translucent to faintly yellowish, the outer tips of the first and second dorsal fins yellow. This is a small species, slender and laterally compressed, growing to a length of around 3-4 cm SL. Named "ivantsoffi" in honour of Walter Ivantsoff, of Macquarie University, Sydney, Australia, in recognition of his valuable contributions to the knowledge of atherinoid taxonomy.
This fish was initially identified as Pseudomugil reticulatus, based on the close resemblance of females from the Timika area to the single known example of that species collected about two kilometres east of Ayamaru (Ajamaru) Lake in the centre of the Vogelkop Peninsula. However, recent collections near the type locality of Pseudomugil reticulatus, which lies some 900 km northwest of Timika, revealed that the two populations are distinctive.