Robert Mertens's day gecko (Phelsuma robertmertensi) is diurnal species of lizard in the family Gekkonidae. The species is endemic to the Comoros.
The specific name, robertmertensi, is in honour of German herpetologist Robert Mertens.
One of the smallest of its genus, P. robertmertensi can reach a total length (including tail) of about 11 centimetres (4.3 in). The body colour can be dark green or bluish green. An orange mid dorsal stripe extends from the head to the tail. The sides of the neck as well as the flanks are greyish brown.
P. robertmertensi inhabits only a small area on the island of Mayotte in the Comoro Islands.
P. robertmertensi is often found on banana trees and in abandoned vanilla orchid plantations.
P. robertmertensi feeds on various insects and other invertebrates. It also licks soft, sweet fruit, pollen and nectar.
P. robertmertensi is oviparous. At a temperature of 28 °C (82 °F), the young will hatch after approximately 49–53 days. The juveniles measure 19–22 millimetres (0.75–0.87 in).
