Inadequate daily oral hygiene care of dental prostheses could worsen general health of frail older people. The index of Augsburger and Elahi is often recommended to assess maxillary dental prosthesis plaque (DPP). To assess the reliability of an expanded and standardised DPP index of Augsburger and Elahi in trained and untrained examiners, and to determine the applicability of the index in community-dwelling frail older people. Ten community-dwelling frail older persons with complete and partial removable dental prostheses (RDPs) (mean ± SD age = 87.9 ± 8.4 years) participated. Seven persons were revisited after 3 months. The index was expanded with mandibular RDPs. Standardisation was achieved with a specially designed reference sheet. The RDPs were photographed after plaque colouring. Two trained examiners scored the photographs; thereafter, two untrained examiners. One trained examiner scored the photographs again after 3 weeks. Changes in DPP over the 3-month period were assessed with Wilcoxon signed-rank test. Inter- and intra-examiner reliability was assessed with intraclass correlation coefficients (ICCs). The DPP scores did not change significantly over time (Z = -0.594, p = 0.553). Inter-examiner reliability was excellent in the trained examiners (ICC = 0.859-0.947), and fair-to-good in the untrained examiners (ICC = 0.671-0.703). Intra-examiner reliability was excellent (ICC = 0.941-0.962). The proposed expansion and standardisation of the DPP index has an excellent inter- and intra-examiner reliability when performed by trained examiners, and a fair-to-good inter-examiner reliability by untrained examiners. Therefore, the index could be recommended for application in dental prostheses-wearing older persons and could be useful for awareness and education of professional and informal caregivers and the older persons themselves. The Netherlands Trail Register NTR6159.