Bananas (banana and plantains) rank sixth among staple food crops (FAO 2018), with production challenged by biotic factors, mainly fungal diseases that may cause a total loss in some orchards (Jones 2018). In April 2017, dieback symptoms (progressive blackening and necrotic aerial plant parts, leaves, fruits, and peduncles) were observed on plantain (Musa AAB subgroup), in Onne, Rivers State, Nigeria (4°42′55.4012″N, 7°10′35.92128″E). Diseased plants (n = 112) were either wilted with blackened necrotic areas, or dead. Nearly 10% of the plants had blackened pseudostems and fruits with slate gray to black internal tissues when sliced, and black, erumpent pycnidia were observed on diseased fruits. A fungal species was always isolated when surface disinfected pieces of diseased samples were cultured on PDA plates. Plates were incubated at 25 ± 2°C for 4 to 15 days to observe conidia. Isolates had colonies and conidia consistent with members of the Botryosphaeriaceae family (Phillips et al. 2013). Immature conidia were single celled, ellipsoidal, and hyaline, and mature conidia were two celled, with a thick wall, a central septum, longitudinal striations, and a dark brown, cinnamon-like color. Mature conidia (n = 20) were 22.9 to 30.0 × 14.2 to 18.4 μm (mean = 27.0 × 15.6 μm). DNA templates of three isolates (23688-2_R16; 19144-18_R15, and PITA_22-1) were amplified using primers ITS1 and ITS4 for the ITS locus and EF1-688F and EF1-1251R for the translation elongation factor 1-α (TEF-1α) locus (Phillips et al. 2013) and sequenced (GenBank accession nos. MZ413346, MZ413347, and MZ413348 for ITS; and MZ420177, MZ420178, and MZ420179 for TEF-1α). A BLASTn query showed 100% identity with reference sequences of various isolates of Lasiodiplodia theobromae. Based on morphological characters and nucleotide homology, the isolates were identified as L. theobromae. To fulfill Koch’s postulates, 4-month-old plants of plantain hybrid PITA 24 and mature fruits from three genotypes (PITA 24, plantain cultivar Obino L’ewai) were inoculated with mycelial plugs from the margins of 5-day-old cultures of the three L. theobromae isolates. Pseudostems were drilled with a sterile 5-mm cork borer, and a mycelial plug was placed into the wound, covered with sterilized cotton, and sealed with Parafilm. Sterile water was injected every third day to maintain moisture at the inoculated area. Toothpicks containing mycelia were used to inoculate fruits in plastic Crisper boxes. Sterile PDA plugs or toothpicks were used for the controls. Inoculated plants and fruits were kept in a screenhouse at room temperature (∼26°C) for 14 days. All inoculated materials developed symptoms similar to the diseased plants in the field. Control plants and fruits remained asymptomatic. L. theobromae was reisolated from the artificially inoculated plant parts and its identity was confirmed. The fungus L. theobromae is distributed in tropical and subtropical regions and has a wide host range (Mehl et al. 2017; Phillips et al. 2013). This fungus was previously reported in gray literature as the causal agent of Musa spp. basal rot at Onne, Nigeria (Mwangi et al. 2005), but its molecular identification was not conducted; it was unknown whether the isolates were indeed L. theobromae or other cryptic species (L. pseudotheobromae or L. parva) (Alves et al. 2008). Over 15 years later, the present study confirms L. theobromae as the causal agent of basal rot of bananas based on nucleotide homology; to our knowledge, this is the first report of L. theobromae causing dieback disease on plantain in Nigeria and in Africa. There is need to conduct more comprehensive distribution surveys and develop appropriate control strategies in Nigeria.