Background <p>The phenomenon of decreased DTI-ALPS index has been reported in patients with chronic insomnia (CI) and has been associated with cognitive decline. Choroid plexus (ChP) volume is closely linked to glymphatic system function. However, the ChP volume in patients with CI and its relationship with both DTI-ALPS index and cognitive function remain unclear.</p> Methods <p>The study included 107 patients diagnosed with CI and 68 healthy controls. All participants underwent brain MRI scans, sleep assessments, and a comprehensive assessment of cognitive function. The ChP volume was automatically segmented from 3D-T1WI and presented as a ratio to total intracranial volume. The DTI-ALPS index was calculated based on DTI and used to evaluate water diffusivity in specific white matter regions.</p> Results <p>Our findings demonstrated significant enlargement of ChP volume and reduction in the DTI-ALPS index among CI patients with declined cognition, compared to patients with normal cognition and healthy controls. The results of partial correlation analysis showed that ChP volume in CI patients with declined cognition was negatively correlated with the Montreal Cognitive Assessment (MoCA) (<i>r</i> = -0.350, <i>P</i> = 0.009) and the Digit Span Test (DST) (<i>r</i> = -0.273, <i>P</i> = 0.044). ChP volume was negatively correlated with DTI-ALPS index (<i>r</i> = -0.342, <i>P</i> = 0.011). Furthermore, DTI-ALPS index was positively correlated with MoCA (<i>r</i> = 0.295, <i>P</i> = 0.029) and DST (<i>r</i> = 0.269, <i>P</i> = 0.047). The results of receiver operating characteristic analysis showed that ChP volume is inadequate as a highly accurate (area under the curve: 0.65, 95% CI: 0.54–0.76, <i>P</i> = 0.009) imaging marker for distinguishing cognitive decline in patients with CI.</p> Conclusions <p>Our study indicated an enlargement of ChP in CI patients with declined cognition. Enlarged ChP in CI patients with declined cognition was associated with lower DTI-ALPS index and cognitive decline.</p>