To identify and clarify definitions and methods of measuring cancer‐related cognitive impairment among prostate cancer patients treated with androgen deprivation therapy (ADT) and to assess the incidence and prevalence of cognitive impairment. A systematic review of Medline, EMBASE, PubMed, PsycINFO and CINAHL up to December 2015 was undertaken to identify English‐language reviews. A total of 28 reviews were identified describing 20 primary studies. There were no studies of incidence. Reported prevalence rates varied between 10% and 69%. Cognitive domains impaired by ADT included: verbal memory, visuospatial ability and executive functions. Cognitive impairment was infrequently defined and four definitions were reported. A variety of measures and methods were used to assess cognitive function including neuropsychological tests, self‐report measures and clinical assessments. The finding that, often, one measure was used to assess more than one aspect of cognition is likely to have contributed to imprecise estimates. There is a need to agree a definition of cognitive impairment in the clinical epidemiology of cancer and to standardise the selection of measures in order to aid accurate assessment and fair comparisons across studies regarding the prevalence of cognitive impairment among prostate cancer patients.