Human development can lead to excessive freshwater discharges, sedimentation, and nutrient inputs in coastal wetlands, which can alter wetland physical conditions such that invasive species become competitively dominant over native species. Festuca perennis (Italian ryegrass) has invaded native salt marsh habitat and established a patchy distribution in San Diego County’s Los Peñasquitos Lagoon, which has experienced substantial freshwater inputs and sedimentation from adjacent urban areas. Salt marsh habitat restoration plans for the Lagoon focus on increasing soil salinity by reestablishing a more natural hydrologic regime, but the ability of restored habitats to resist future F. perennis invasion is unknown. Two experiments were designed to study the influences of various abiotic and biotic factors on F. perennis invasion in the Lagoon. A greenhouse experiment used microcosms to examine how soil moisture and soil salinity affected ryegrass seedling survival and growth. In a field experiment, F. perennis seeds were planted in cleared and uncleared areas within F. perennis patches as well as patches containing native salt marsh species (predominately Frankenia salina and Salicornia pacifica). Vegetative growth and various abiotic conditions were monitored throughout the field experiment. During the greenhouse experiment, total mortality of F. perennis seedlings occurred after exposure to salinities of 19 mg/g water (parts per thousand [ppt]), although salt shock and heat stress may have exacerbated seedling salt sensitivity. In the field experiment, both ryegrass and native species had a home-field advantage in their respective patches. The lack of ryegrass growth in intact stands of native species provided evidence of interspecific competition between ryegrass and native species. Additionally, high soil salinities significantly decreased ryegras growth, and sites located north of the Lagoon’s railroad berm had significantly less native species growth. Although this study has revealed the importance of both biotic and abiotic factors in understanding F. perennis invasion, further experimentation and adaptive management are necessary to ensure Lagoon restoration success.