Background and aims – The Arthoniaceae form a species-rich family of lichenized, lichenicolous and saprophytic fungi in the order Arthoniales. As part of taxonomic revisions of the African Arthoniaceae, a number of species assignable to the genus Synarthonia were collected and sequenced. The present study aims at placing the genus in a phylogeny for the first time and at clarifying its circumscription. Methods – Nuclear (RPB2) and mitochondrial (mtSSU) DNA sequences from freshly collected specimens were obtained and analysed with phylogenetic Bayesian and maximum likelihood (ML) methods. Key results – Synarthonia is closely related to the genera Reichlingia and Coniocarpon in the Arthoniaceae. Six Synarthonia species are described as new to science and ten new combinations into this genus are made. A worldwide identification key to the genus Synarthonia is provided. Lectotypes are chosen for Arthonia elegans, A. inconspicua, A. lopingensis, A. ochracea, A. subcaesia and A. translucens. Arthonia thamnocarpa is synonymized with Sclerophyton elegans, and Arthonia elegans with Coniocarpon fallax. Synarthonia ochracea is shown to be a misunderstood species in the past and recent literature, since it was erroneously synonymized with Coniocarpon elegans. Synarthonia ochracea appears to start its life cycle as a non-lichenized lichenicolous fungus on Graphis before developing a lichenized thallus or it might be a facultatively lichenicolous fungus. It belongs to a complex of closely related species whose biology and circumscription are still in need of further studies.Conclusions – Synarthonia forms a monophyletic but somewhat heterogeneous lineage closely related to Coniocarpon and Reichlingia. As delimited here, Synarthonia includes corticolous lichens with a trentepohlioid photobiont as well as non-lichenized lichenicolous fungi. The core group is characterized by white pruinose ascomata, but species producing orange pruinose or non-pruinose ascomata are also included. Ascospores are transversely septate with an enlarged apical cell or are muriform. Future molecular and morphological studies are needed for a better circumscription and definition of the genus.