<p>Dissolved oxygen (DO) strongly regulates stream-dwelling fish physiology, yet acute responses to oxygen fluctuations remain poorly understood. We investigated the effects of hypoxia–reoxygenation on juvenile <i>Acrossocheilus fasciatus</i>, a mountain stream cyprinid. Fish were exposed to 24&#xa0;h of hypoxia (2.40 ± 0.20&#xa0;mg L⁻¹ DO) followed by 24&#xa0;h of reoxygenation (7.00–8.00&#xa0;mg L⁻¹ DO), and responses were assessed at morphological, endocrine, biochemical, and molecular levels. Hypoxia induced pronounced gill remodeling, characterized by elongation and thinning of protruding lamellae, increased inter-lamellar spacing, and changes in interlamellar cell mass (ILCM), collectively indicating expansion of the respiratory surface area. These structural changes showed only partial reversal during reoxygenation. Plasma cortisol levels increased under prolonged hypoxia and remained elevated into reoxygenation. Hepatic antioxidant-related parameters exhibited time-dependent but non-uniform changes: Superoxide dismutase (SOD) activity declined significantly during early hypoxia, whereas catalase (CAT) activity decreased and glutathione (GSH) levels increased at the same stage, with most parameters returning toward control levels during reoxygenation. In contrast, malondialdehyde (MDA) accumulated persistently and increased further during reoxygenation, indicating ongoing lipid peroxidation. At the molecular level, <i>hif-1α</i> and <i>ampk</i> were upregulated during hypoxia, consistent with activation of hypoxia sensing and energy stress pathways. Expression of <i>bcl-2</i> decreased sharply at 12&#xa0;h, indicating reduced anti-apoptotic restraint, whereas <i>foxo1a/3a</i> were upregulated from 12&#xa0;h onward, supporting the onset of gill structural adjustment and tissue repair during hypoxia–reoxygenation. Ecologically relevant hypoxia in stream systems elicits active physiological adjustment in fishes, yet repeated events may pose substantial long-term risks.</p>