<p>Sedation during mechanical ventilation is common in intensive care units (ICUs). Functional near-infrared spectroscopy (fNIRS) has shown potential in monitoring brain function. This study aimed to evaluate and compare the brain functional connectivity (FC) characteristics in different sedated patients using fNIRS. Thirty sedated patients in the ICU and 16 conscious, non-sedated patients from the emergency ward were enrolled. Sedated patients were assigned to insufficient sedation (+ 1 to + 4), light sedation (-2 to 0), or deep sedation (-5 to -3) groups based on the Richmond Agitation Sedation Scale. Six-minute resting-state light intensity signals were collected using fNIRS and converted into a time series of relative oxyhemoglobin concentrations. FC characteristics of the overall brain and six regions of interest (ROIs) were compared using Pearson’s correlation coefficients calculation. Multiple comparisons were corrected using the False Discovery Rate method. The deep sedation group had the highest overall mean FC compared with the other groups, which may be associated with the alterations of sedatives on cortical activity. ROI-level analyses revealed higher FC in the prefrontal, parietal, and occipital cortices in the deep sedation group than in the other groups. The deep sedation group had higher channel-channel and ROI-ROI FC in the FC matrices than the other groups. ROI-ROI connections differences were observed between the deep sedation and insufficient sedation groups. No significant difference in FC was identified between the light sedation group and the conscious control group. Deep sedation exhibited the highest overall and regional FC, and light sedation and the conscious state had similar FC patterns, supporting the potential utility of fNIRS for sedation monitoring in the ICU setting.</p><p><i>Trial registration</i>: ChiCTR2300068437 registration May 15, 2023, <a href="https://clinicaltrials.gov/ct2/show/ChiCTR2300068437">https://clinicaltrials.gov/ct2/show/ChiCTR2300068437</a></p>