A six-week feeding trial was conducted to evaluate the effects of dietary supplementation with copper nanoparticles (CuNPs) administered at 6.5 or 13.0 mg Cu/kg diet, in combination with different types of dietary fibre, on haematological and immune parameters in rats. Ten experimental groups were included. Two control groups received cellulose-based diets containing copper(II) carbonate(CuCO3) at 6.5 or 13.0 mg Cu/kg diet. In the experimental groups, CuCO3 was replaced with CuNPs at the corresponding concentrations, and diets were supplemented with various fibre types: cellulose, pectin, inulin, or psyllium. At the end of the experimental period, blood samples were collected for the assessment of haematological and immune indices. Irrespective of Cu form or dose, diets containing pectin or inulin lowered the counts of white blood cells (WBC) and lymphocytes (LYM), while inulin or psyllium reduced interleukin 6 (IL-6) concentrations. In contrast, CuNPs administered at 6.5 mg Cu/kg diet in combination with psyllium increased WBC and LYM counts. The inclusion of inulin or psyllium in diets containing the lower CuNPs dose decreased immunoglobulin M (IgM), IL-6, and tumour necrosis factor α (TNF-α) levels. Replacing CuCO3 with CuNPs at 6.5 mg Cu/kg in diets containing cellulose, inulin, or psyllium lowered immunoglobulin A (IgA) levels, while cellulose additionally increased C-reactive protein (CRP) concentrations. A diet containing double dose of CuNPs (13.0 mg Cu/kg) with psyllium reduced mean corpuscular haemoglobin (MCH) and increased red cell distribution width (RDWc) and mean platelet volume (MPV). Moreover, high-dose CuNPs diets containing pectin, inulin, or psyllium lowered IgA, IgM, interleukin 2 (IL-2), and TNF-α concentrations, and pectin additionally reduced immunoglobulin G (IgG) levels. In summary, replacing standard CuCO3 with CuNPs, even at 6.5 mg Cu/kg diet, induced inflammation and impaired immune function in rats. However, supplementation with pectin or inulin alleviated the adverse immune and inflammatory effects caused by Cu nanoparticles.