In 2021, a 10-month-old olive (Olea europaea L.) orchard of cv. Arbequina located in Albatana (Albacete, Castilla-La Mancha, Spain), growing in super-high-density showed foliar chlorosis, twig defoliation, wilt, dieback, internal wood discoloration and general decline in 20% of 10.000 trees surveyed. Fast growing fungal colonies were consistently isolated on antibiotic PDA. Two representative isolates (21014-3; 21014-15) were selected. The colonies were flat, with abundant aerial mycelium, white to beige with olivaceous gray patches. Conidia were brown, smooth, granular, globose to elongate ellipsoid (average 8.14 × 6.5 μm; n = 30 per isolate). They were identified by morphology as Arthrinium sp. (Samuels et al. 1981). The TUB and EF gene regions were sequenced with primer pairs T1/Bt2b and EF1-728 F/EF1-986R, respectively. Sequences were deposited in GenBank (Accession Nos.: TUB: OM417231, OM417232; EF: OM417229, OM417230). BLAST showed high identity (> 99%) with the reference sequences of Apiospora marii KF144992 (TUB) and MK017961 (EF). A multilocus alignment was used to confirm their identity by phylogenetic analysis. The two isolates were identified as Apiospora marii (anamorph Arthrinium marii). Pathogenicity tests were conducted on 9-month-old olive plants (‘Arbequina’), by dipping the roots overnight in a conidial suspension (105 conidia/ml), and planted in 0.7 l plastic pots containing sterile peat moss (n = 12 per isolate). Plants dipped in sterile distilled water were included as control (n = 12). Chlorotic leaves, wilt and internal wood discoloration were observed on inoculated plants at 3 months after inoculation. No symptoms were observed on controls. The pathogen was consistently reisolated from the basal stem and roots of inoculated plants. Apiospora marii has been reported in olive in Italy, causing tree dieback (Gerin et al. 2020). To our knowledge, this is the first report of A. marii causing wilt, dieback and tree decline in olive in Spain (Farr and Rossman 2022).