<p>The photoperiod, an essential environmental factor influencing the growth, reproduction, and physiological functions of teleosts, exhibits effects that are highly species-specific. This study investigates the impact of five distinct photoperiod treatments on the growth performance, gonadal development, and hypothalamic-pituitary–gonadal (HPG) axis regulation in <i>Phoxinus lagowskii</i>, a high-latitude cold-water fish with significant aquaculture potential. Through comprehensive analyses of growth indices, serum sex hormone levels, gonadal histology, and gene expression, we demonstrate that photoperiod significantly affects the physiology of <i>P. lagowskii</i>. Females exhibited optimal growth and gonadal development under a 16L:8D photoperiod, with increased expression of key genes (<i>gnrh2</i>, <i>gnrh3</i>, <i>kiss1</i>, <i>kiss2</i>, <i>cyp19a1</i>, and <i>foxl2</i>) and elevated levels of gonadotropins (Fsh, Lh) and estradiol (E₂). In contrast, males showed enhanced reproductive performance under an 8L:16D photoperiod, characterized by higher testosterone (T) levels and upregulated expression of <i>dmrt1</i> and <i>sox9a</i>. The inverse relationship between the hepatosomatic index (HSI) and gonadosomatic index (GSI) suggests an energy trade-off between hepatic energy reserves and reproductive investment.&#xa0;Moreover, melatonin further regulates the HPG axis by affecting the synthesis and release of GnRH and kisspeptins, highlighting its role in mediating photoperiodic effects on reproduction. These findings underscore the sex-specific responses of <i>P. lagowskii</i> to photoperiod and provide critical insights into the physiological mechanisms underlying photoperiod regulation in cold-water fish. This study offers practical strategies for optimizing aquaculture practices to enhance growth and reproductive efficiency.</p>