Amphipods are common, key grazers in coastal macroalgal communities. Understanding their feeding habits will help us predict the fate of primary producers in macroalgal-based ecosystems and the role of amphipods in the detritus and grazing food chains. The feeding habits of amphipods are frequently reported;however, these data are primarily obtained from laboratory experiments. Considering that amphipod feeding selectivity is influenced by various factors such as habitat, food availability, morphological traits, and nutritional value of food sources, differences between laboratory and field experiments are expected. This study measured stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N, respectively) of two dominant amphipod species, Ampithoe valida and Corophium uenoi, and their potential food sources in Yundang Lagoon, and analyzed spatial variation in their diet. Potential amphipod food sources contained particulate organic matter (POM), sedimentary organic matter (SOM), and Ulva lactuca and its associated epiphytes. The δ¹³C values ranged from -24.0‰ for POM to -11.8‰ for U. lactuca, and the corresponding δ¹⁵N values from -1.7‰ to 4.7‰. There were no significant differences between the δ¹³C values of U. lactuca and POM;however, the δ¹³C values for sedimentary organic matter (SOM) and U. lactuca epiphytes were higher in the outer lagoon than the inner lagoon. The difference in δ¹³C for SOM between the inner and outer lagoon was mainly explained by spatial shifts in the sources of organic matter, such as POM sedimentation before macroalgal blooms in the inner lagoon and organic debris from decaying U. lactuca in the outer lagoon. Epiphytes mainly contained benthic microalgae, bacteria, and organic debris, so variation in δ¹³C may be caused by spatial shifts in epiphyte composition. Other than POM, the δ¹⁵N values of the carbon sources were higher in the outer lagoon than in the inner lagoon, and this is largely attributed to differences in the nitrogen sources that are absorbed. This inference is also supported by the spatial variation in δ¹⁵N for phytoplankton cultivated in the laboratory using filtered seawater collected from the Yundang Lagoon. Spatial dietary variation was not obvious for A. valida, which mainly obtained carbon from U. lactuca and associated epiphytes. However, significant spatial variation in diet was observed for C. uenoi, which mainly fed on epiphytes attached to U. lactuca and POM in the inner lagoon, but U. lactuca and associated epiphytes in the outer lagoon. Our results showed that the δ¹⁵N content in amphipods was much higher in the outer lagoon in than the inner lagoon, and this variation is largely caused by spatial variation in food sources.