Disentangling Parkinson’s disease (PD) and Progressive Supranuclear Palsy (PSP) may be a diagnostic challenge. Cognitive signs may be useful, but existing screens are often insufficiently sensitive or unsuitable for assessing people with motor disorders. We investigated whether the newly-developed ECAS, designed to be used with people with even severe motor disability, was sensitive to the cognitive impairment seen in PD and PSP, and able to distinguish between these two disorders. 30 patients with PD, 11 patients with PSP and 40 healthy controls were assessed using the ECAS, as well as an extensive neuropsychological assessment. The ECAS detected cognitive impairment in 30 % of the PD patients, all of whom fulfilled diagnostic criteria for mild cognitive impairment. The ECAS was also able to detect cognitive impairment in PSP, with 81.8 % of patients performing in the impaired range. ECAS total score distinguished between PSP and healthy controls with high sensitivity (91.0) and specificity (86.8). Importantly, the ECAS was also able to distinguish between the two syndromes, with the measures of verbal fluency offering high sensitivity (82.0) and specificity (80.0). In sum, the ECAS is a quick, simple and inexpensive test that can be used to support the differential diagnosis of PSP.