Nanofibrillar cellulose (NFC) hydrogel has emerged as a promising implantable material for therapeutic applications. In this study, the relatively longer-lived positron-emitting radionuclide zirconium-89 was chelated to the octadentate deferoxamine (DFO*)-conjugated NFC hydrogel ([⁸⁹Zr]Zr-DFO*-NFC) to enable longitudinal monitoring of its in vivo fate using positron emission tomography techniques. Following subcutaneous implantation in healthy mice, [⁸⁹Zr]Zr-DFO*-NFC retained radioactivity at the implant site for at least 14 days, with minimal signal detected in the kidneys, urinary bladder, and overlying skin. In contrast, mice receiving a control formulation of [⁸⁹Zr]Zr-oxalate mixed with NFC hydrogel showed progressive accumulation of radioactivity in the bones, consistent with known [⁸⁹Zr]Zr-oxalate distribution patterns, and only limited retention at the implant site by Day 7. These findings demonstrate that [⁸⁹Zr]Zr-DFO*-NFC hydrogel implants exhibit high in vivo stability with negligible systemic release following subcutaneous implantation, supporting their potential use as safe and traceable biomaterial platforms.