Carboxylated multi-walled carbon nanotubes (MWCNT-COOH) exerts strong adsorption capacity for pentachlorophenol (PCP) and they inevitably co-occur in the environment, but few studies have characterized the effects of MWCNT-COOH on the bioavailability of PCP and its oxidative and tissue damages to fish. In this work, we assessed the PCP accumulation in different organs and the induced oxidative and tissue damages of goldfish following 50-d in vivo exposure to PCP alone or co-exposure with MWCNT-COOH. Our results indicated that PCP bioaccumulation in goldfish liver, gill, muscle, intestine and gut contents was inhibited after co-exposure with MWCNT-COOH in uptake phase. PCP exposure alone and co-exposure with MWCNT-COOH evoked severe oxidative and tissue damages in goldfish bodies, as indicated by significant inhibition of activities of antioxidant enzymes, remarkable decrease in glutathione level, simultaneous elevation of malondialdehyde content, and obvious histological damages to liver and gill. The decreased accumulation of PCP in the presence of MWCNT-COOH led to the reduction of PCP-induced toxicity to liver tissues, as confirmed by the alleviation of hepatic oxidative damages. However, co-exposure groups had higher concentrations of PCP in the tissues than PCP treatment alone (p < 0.05 each) in the depuration phase, revealing that MWCNT-COOH-bound pollutants might pose higher risk once desorbed from the nanoparticles. These results provided substantial information regarding the combined effects of PCP and MWCNT-COOH on aquatic species, which helps to deeply understand the potential ecological risks of the emerging pollutants.