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In mammals the melanocortin 4 receptor (Mc4r) signaling system has been mainly associated with the regulation of appetite and energy homeostasis. In fish of the genus <i>Xiphophorus</i> (platyfish and swordtails) puberty onset is genetically determined by a single locus, which encodes the <i>mc4r</i>. Wild populations of <i>Xiphophorus</i> are polymorphic for early and late-maturing individuals. Copy number variation of different <i>mc4r</i> alleles is responsible for the difference in puberty onset. To answer whether this is a special adaptation of the Mc4r signaling system in the lineage of <i>Xiphophorus</i> or a more widely conserved mechanism in teleosts, we studied the role of Mc4r in reproductive biology of medaka (<i>Oryzias latipes</i>), a close relative to <i>Xiphophorus</i> and a well-established model to study gonadal development. To understand the potential role of Mc4r in medaka, we characterized the major features of the Mc4r signaling system (<i>mc4r, mrap2, pomc, agrp1</i>). In medaka, all these genes are expressed before hatching. In adults, they are mainly expressed in the brain. The transcript of the receptor accessory protein <i>mrap2</i> co-localizes with <i>mc4r</i> in the hypothalamus in adult brains indicating a conserved function of modulating Mc4r signaling. Comparing growth and puberty between wild-type and <i>mc4r</i> knockout medaka revealed that absence of Mc4r does not change puberty timing but significantly delays hatching. Embryonic development of knockout animals is retarded compared to wild-types. In conclusion, the Mc4r system in medaka is involved in regulation of growth rather than puberty.