Based on collection activities over the past 25 years, a revision of literature, and unpublished materials, an annotated checklist of trees and arborescent species found in and reported from El Salvador, Central America is presented. This first of two volumes covers 58 angiosperm plant families (Acanthaceae to Lythraceae). A total of 577 species and 55 subspecies and forms are recognised, representing 583 taxa at the most basic rank, of which 486 are native in El Salvador. The checklist includes the correct names, their synonyms as far as they have been in use for Salvadorian taxa, the basionyms, and nomenclatural details for all names cited. Additional data for each taxon include vernacular names, data on local and global distribution, selected information on (potential) utilization, as well as detailed literature references from works directly referring to the flora of El Salvador as well as a section of relevant Central American floras. A total of 6807 herbarium collections are cited. of which 5745 specimens are located – and have been revised – at LAGU. The introduction provides a summary of collection activities related to that institution as well as an introduction to taxonomic inventories in El Salvador.