Fibrosis is characterized by a pathologic deposition of collagen I, leading to impaired function of organs. Tissue biopsy is the gold standard method for the diagnosis of fibrosis but this is an invasive procedure, subject to sampling errors. Several non-invasive techniques such as magnetic resonance imaging (MRI) using non-specific probes have been developed but they are not fully satisfying as they allow diagnosis at a late stage. In this study, collagelin, a collagen-binding peptide has been covalently linked using click chemistry to pegylated Ultra Small Super Paramagnetic Iron Oxide Nanoparticles (USPIO–PO–PEG–collagelin NPs) with the aim of diagnosing fibrosis at an early stage by MRI. USPIO–PO–PEG–collagelin NPs showed a high affinity for collagen I, two times higher than that of free collagelin whereas not peptide labeled USPIO NPs (USPIO–PO–PEG-yne) did not present any affinity. NPs were not toxic for macrophages and fibroblasts. Diffusion through collagen hydrogels concentrated at 3 and 10 mg mL−1 revealed a large accumulation of USPIO–PO–PEG–collagelin NPs within the collagen network after 72 hours, ca. 3 times larger than that of unlabeled USPIO, thereby evidencing the specific targeting of collagen I. Moreover, the quantity of USPIO–PO–PEG–collagelin NPs accumulated within hydrogels was proportional to the collagen concentration. Subsequently, the NPs diffusion through collagen hydrogels was monitored by MRI. The MRI T2 time relaxation decreased much more significantly with depth for USPIO–PO–PEG–collagelin NPs compared to unlabeled ones. Taken together, these results show that USPIO–PEG–collagelin NPs are promising as effective MRI nanotracers for molecular imaging of fibrosis at an early stage.