Technical Abstract: The Jepson Manual; Higher Plants of California is the authoritative floristic reference for the state, that has rapidly become out of date given substantial advances in plant systematics and the arrival of new exotic species. Recent aggressive spread of Ludwigia species (Onagraceae) has impacted sensitive wetlands, water conveyance canals, and rice fields in Pacific western states. Variable growth responses to environmental conditions and contradictory taxonomic keys have complicated accurate species identifications that are essential for weed management. Here we present a revised taxomomic treatment for Ludwigia (Onagraceae) found in California. The updated taxonomic descriptions and key are based on extensive biosystematics research, and considers currently available cytological, morphometric and molecular data. The treatment describes six Ludwigia taxa in California, while previously only five were recognized. The treatment now includes the recently introduced invasive weed, L. grandiflora. L. hexapetala and L. peploides taxa previously thought to be native to California are now recognized as naturalized and disjunct from their native South American range.