Sydowiellaceae is a poorly studied family of the order Diaporthales, comprising a collection of morphologically diversified taxa. Eleven genera have been previously listed under this family. In this study, we provide a DNA sequence-based phylogeny for genera of Sydowiellaceae based on analyses of a combined LSU, ITS, RPB2 and TEF sequence dataset to establish the boundaries within the family. We introduce, describe, and illustrate three new genera, Italiomyces, Ranulospora, and Tenuiappendicula, based on newly collected specimens. Another three new genera, Alborbis, Breviappendix, and Paragnomonia are introduced to accommodate Cryptodiaporthe galericulata, Gnomonia rosae, G. rostellata, G. rubi, and G. fragariae. Alborbis galericulata comb. nov., Breviappendix rubi comb. nov., B. rosae comb. nov., B. rostellata comb. nov. and Paragnomonia fragariae comb. nov. are proposed. Two new species, Sydowiella centaureii and Sillia karstenii, are introduced with descriptions and illustrations. Winterella is replaced with a new genus Tortilispora typified by T. aurantiaca. Three new combinations are proposed in Tortilispora. We synonymize Stegophora under Lambro based on morphological similarities. An updated key for genera of Sydowiellaceae is presented.