The present study aimed to scrutinize the hypothesis that the dietary addition of fiber with functional properties would prevent the thwarting effects of copper nanoparticles (Cu-NP) on fecal microbial metabolic activity. The Wistar rats were fed for 6 weeks with diets with two different contents of Cu-NP (a recommended dose of 6.5 mg/kg or doubled) and four sources of dietary fiber: control – cellulose; and experimental – pectin, inulin, and psyllium. The activity of bacterial enzymes and short-chain fatty acids (SCFA) production were analyzed in the excreted feces collected on subsequent days of feeding. The inclusion of Cu-NP to the diet, especially in the higher dose tested, resulted in a rapid reduction in the fecal microbial enzymatic activity after only the first day of feeding. The addition of functional fiber to diets containing Cu-NP enhanced bacterial α-glucosidase, β-glucosidase, β-glucuronidase, α-arabinofuranosidase activity and SCFA production vs. diets with control cellulose. That effect was most rapidly evident with pectin, while the effect of inulin or psyllium addition exceeded that of pectin only in some cases, i.e., α-glucosidase activity as well as content of propionic and butyric acids. In conclusion, the high potential of dietary functional fiber in reducing the suppressive effect of Cu-NP on the intestinal microbiota activity should be recognized.