This is the first report of the occurrence of core rot on peaches [Prunus persica (L.) Batsch] caused by the fungus Alternaria alternata in Greece. This disease caused significant preharvest (at commercial ripening stage) and postharvest damage (at a percentage of ~5% of the total production) in the cultivar ‘Fayette’. Rotting of the core began while fruit were still on the tree, with a soft, wet, brown rot progressing in the flesh around the stone. A. alternata was isolated on acidified potato dextrose agar from the edges of the rotted tissues. Symptoms were reproduced under laboratory conditions by injection of spore suspensions in the flesh of mature and immature fruit of ‘Fayette’. In contrast, immature fruit did not show core rot symptoms when naturally infected. Furthermore, none of the other peach cultivars tested (‘Spring Lady’, ‘Spring Crest’, ‘June Gold’, ‘Red Haven’, ‘Sun Crest’, ‘Sun Cloud’) showed the symptoms of core rot of fruit at the commercial ripening stage. The rates of infection were significantly reduced at 10°C and completely inhibited at 2–4°C. The effectiveness of the fungicides PIΛAZIN 60WP, Thiophanate methyl 70WP, Folicur 25WG, Rovral 50WP, Dithane M-45 80WP and Switch 25/37.5 WP, at rates recommended by the manufacturers, were evaluated against A. alternata on agar and artificially inoculated fruit. The fungicides Folicur 25WG, Rovral 50WP and Switch 25/37.5 WP significantly reduced the development of A. alternata. Moderate effectiveness was shown by Dithane M-45 80WP. In contrast, the fungicides PIΛAZIN 60WP and Thiophanate methyl 70WP were not effective against this pathogen.