In the current study, we aim to measure T1rho (T (1ρ)) in the hippocampus in the brain of control, Alzheimer's disease (AD), Parkinson's disease (PD), and PD patients with dementia (PDD), and to determine efficacy of T (1ρ) in differentiating these cohorts. With informed consent, 53 AD patients, 62 PD patients, 11 PDD patients, and 46 age-matched controls underwent a standardized clinical assessment including mini-mental state examination (MMSE) and brain T (1ρ) MRI on a 1.5-T clinical-scanner. T(1ρ) maps were generated by fitting each pixel's intensity as a function of the spin-lock pulse duration. In control, AD, PD and PDD, mean ± SE T (1ρ) values in the right hippocampus (RH) were 92.15 ± 2.00, 99.65 ± 1.98, 85.68 ± 1.87, 102.47 ± 4.66 ms while in the left hippocampus (LH) these values were 90.16 ± 1.82, 99.53 ± 1.91, 84.33 ± 2.03, 95.33 ± 4.64 ms. Significant difference for both RH and LH T (1ρ) across the groups (p < 0.001) was observed. Both RH and LH T (1ρ) were significantly increased in AD compared to control (p = 0.034, p = 0.001) and PD (p < 0.001, p < 0.001). In control, both RH and LH T (1ρ) values were significantly increased compared to PD (p = 0.031, p = 0.027) while compared to PDD only the RH T (1ρ) value was significantly decreased (p = 0.043). Both RH and LH T (1ρ) values in PD were significantly lower than PDD (p = 0.004, p = 0.032). No significant correlation between the T (1ρ) and age as well as between T (1ρ) and MMSE scores was observed. The serial measurement of T(1ρ) in both AD and PD may provide the nature of disease progression and may contribute to their early diagnosis.