Sandy beaches and their associated surf zones support productive food webs that are structured by subsidies from adjacent marine ecosystems. Located at the interface of land and sea, beach ecosystems are characterized by highly dynamic intertidal zonation of mobile macroinvertebrates that varies across time scales ranging from daily and semilunar shifts with tides, to strongly seasonal patterns of beach condition and productivity. Sandy beaches provide an ideal system to examine temporal variation in resource dynamics, an important but understudied dimension of food webs. The barred surfperch, Amphistichus argenteus, is an abundant and widespread surf zone fish in central and southern California that represents an important sport fishery. This live-bearing fish forages on sandy beaches and has been reported to rely primarily upon the intertidal sand crab, Emerita analoga, as prey. However, little is known about the breadth of prey types used by this fish, especially across temporal variation in the supply and accessibility of prey resources on sandy beaches. To address this gap, we investigated variation in the diet of barred surfperch on two temporal scales, season and tide phase, that are known to affect the dynamic distribution of their intertidal prey resources on sandy beaches. Beaches exhibit strong seasonal variation in habitat characteristics that are reflected in populations of intertidal prey for surfperch. We hypothesized that the diet of barred surfperch would respond to seasonal shifts in beach condition and the availability of intertidal macroinvertebrate prey. We also expected that diet could differ with life stage of the fish. To evaluate these questions, we investigated the effect of season on the diet of juvenile and adult barred surfperch, macroinvertebrate prey composition and availability, and relevant environmental variables for two years at Refugio beach. Community composition and the availability of key intertidal prey taxa varied significantly with season. The lower beach crab, Emerita analoga, was important in the diet of barred surfperch and dominated the diet of adult fish in every season, accounting for 92% of their diet. In contrast, juvenile barred surfperch demonstrated strong seasonality in their diet and foraged on prey resources from a wider range of microhabitats, extending from subtidal to upper intertidal zones. The prey resources used by juvenile fish varied with seasonal changes in beach physical characteristics, the population dynamics of key prey species, and species richness of the intertidal macroinvertebrate community. Seasonality in the diet of juvenile fish reveals the dynamic and diverse food web resources supporting populations of barred surfperch. Semilunar tide phases greatly affect the inundation of intertidal habitats and the distribution of invertebrates on beaches. We hypothesized that the diet of barred surfperch would differ among semilunar tide phases. Specifically, we expected a greater diversity of beach invertebrates would be present in surfperch diet during spring tides that sweep more of the intertidal beach face than during neap tides. To answer this question, we investigated the diet of barred surfperch and the availability of beach prey across semilunar shifts in tidal inundation to explore how fish access prey from a range of intertidal zones at R Beach. Community composition of intertidal invertebrates did not vary with tide phase. Overall, species richness in surfperch diet was positively correlated with species richness of prey on the beach. Increased access to intertidal zones during spring tides increased the diversity of prey used by adult and juvenile barred surfperch, suggesting semi lunar tidal inundation is a mechanism of trophic connectivity across sandy beaches and surf zones. During spring tides, barred surfperch diet included intertidal clams, upper intertidal talitrid amphipods, and a reduced dominance of sand crabs. Diet composition varied significantly with fish life stage through shifts in the use of key prey taxa, supporting our finding of seasonal ontogenetic shifts in diet and underscoring the role of life stage in the food web dynamics of live bearing surf zone fish like surfperch. Barred surfperch and the intertidal communities that support them reside at a narrow intersection of human impacts and encroaching seas. Sandy beach ecosystems are highly vulnerable to the effects of sea level rise and other manifestations of climate change. These growing threats to beaches and the food webs they support are already being compounded by anthropogenic disturbance and infrastructure at many urban beaches, impacting intertidal biodiversity and beach habitat quality and quantity. Our findings provide new information on the range of prey resources and the dynamics of the intertidal food webs that support populations of this highly valued surf zone fish. Increased understanding of these trophic connections between beaches and surf zones is needed for the challenges facing coastlines in the coming decades.