Studies have suggested that, in subjects with subjective cognitive impairment (SCI), Alzheimer's disease (AD)‐like changes may occur in the brain. Recently, an in vivo study has indicated the potential of ultra‐high‐field MRI to visualize amyloid‐beta (Aβ)‐associated changes in the cortex in patients with AD, manifested by a phase shift on T2*‐weighted MRI scans. The main aim of this study was to investigate whether cortical phase shifts on T2*‐weighted images at 7 T in subjects with SCI can be detected, possibly implicating the deposition of Aβ plaques and associated iron. Cognitive tests and T2*‐weighted scans using a 7‐T MRI system were performed in 28 patients with AD, 18 subjects with SCI and 27 healthy controls (HCs). Cortical phase shifts were measured. Univariate general linear modeling and linear regression analysis were used to assess the association between diagnosis and cortical phase shift, and between cortical phase shift and the different neuropsychological tests, adjusted for age and gender. The phase shift (mean, 1.19; range, 1.00–1.35) of the entire cortex in AD was higher than in both SCI (mean, 0.85; range, 0.73–0.99; p < 0.001) and HC (mean, 0.94; range, 0.79–1.10; p < 0.001). No AD‐like changes, e.g. increased cortical phase shifts, were found in subjects with SCI compared with HCs. In SCI, a significant association was found between memory function (Wechsler Memory Scale, WMS) and cortical phase shift (β = –0.544, p = 0.007). The major finding of this study is that, in subjects with SCI, an increased cortical phase shift measured at high field is associated with a poorer memory performance, although, as a group, subjects with SCI do not show an increased phase shift compared with HCs. This increased cortical phase shift related to memory performance may contribute to the understanding of SCI as it is still unclear whether SCI is a sign of pre‐clinical AD.