El Niño conditions are known to alter ocean circulation patterns causing a reduction in upwelling events that leads to warmer water with reduced concentrations of inorganic nutrients in coastal waters. The 2015/16 El Niño event resulted in significant losses of the habitat forming giant kelp, Macrocystis pyrifera, with concurrent increases in the non-native, invasive brown alga Sargassum horneri across San Diego, CA. M. pyrifera is known to be highly sensitive to both temperature and nutrient availability, however much less is known about the response of S. horneri to these conditions. The goals of this study were to investigate if and how El Niño-like conditions may have facilitated the invasion of S. horneri in southern CA using both laboratory and field-based approaches. In laboratory experiments, S. horneri maintained positive growth in conditions well exceeding M. pyrifera’s thermal tolerance limits. Temperature was the main driver of growth rates for S. horneri juveniles, but nitrate concentration affected other measurements of algal physiological performance, such as photosynthetic efficiency/maximum quantum yield and pigment concentration. Further, in the field, S. horneri displayed strong seasonality, but with contrasting timing to native M. pyrifera suggesting that the phenology of these two species may be offset. Given the rise in sea surface temperatures and predicted increases in the frequency and intensity of El Niño and marine heat waves, these findings suggest there is increased risk of further invasion by the non-native S. horneri within southern California’s iconic giant kelp forests, but more research is needed to evaluate future trends.