OBJECTIVE: The loss of locus coeruleus (LC)-norepinephrine system may contribute to freezing of gait (FOG) in Parkinson's disease (PD), but free-water (FW) imaging has not been applied to investigate LC microstructural degeneration in FOG. This study was to investigate the role of the LC-norepinephrine system in FOG pathophysiology using FW imaging and resting-state functional magnetic resonance imaging. METHODS: FW metrics of LC were analyzed in 52 healthy controls, 79 PD patients without FOG (Non-FOG), and 110 PD patients with FOG (48 "Off-period" FOG and 62 "Levodopa unresponsive" FOG). Correlation between LC FW metrics and clinical scales were assessed. Functional connectivity analysis with LC as the region of interest was performed across groups during medication withdrawal. Structural and functional differences in LC between FOG subgroups and the effects of dopaminergic medication were also explored. RESULTS: FOG patients had increased FW value, FW-corrected mean diffusivity, axial diffusivity, and radial diffusivity in LC, and decreased FW-corrected fractional anisotropy compared to Non-FOG patients and healthy controls. In FOG patients, FW value and FW-corrected mean axial diffusivity were positively correlated with the new FOG questionnaire scores. LC functional connectivity with occipital regions was reduced in FOG patients. No significant differences in LC microstructure or functional connectivity were observed between FOG subgroups during their "OFF" state. In contrast to "Levodopa-unresponsive" FOG patients, oral medication significantly improved LC functional connectivity with occipital regions in "Off-period" FOG patients. CONCLUSIONS: LC degeneration may disrupt motor and compensatory network integration, especially in visual-motor pathways, contributing to FOG.