Social anxiety occurs in everyday social interactions, yet the real-world factors that shape the moment-to-moment experience of social anxiety have not been fully explored. Using ecological momentary assessments (smartphone-based, five signals a day for 21 days), the present study examined the associations between state social anxiety (SSA) and characteristics of interaction partners in varied contexts, and how these momentary associations differed with trait social anxiety (TSA). Ninety-two participants (54% female, age from 18 to 34) completed 4185 momentary reports. Results from (...) multilevel models showed that perceived judgmentalness and unfamiliarity of interaction partners were positively associated with SSA, and the associations were stronger for the high TSA group (n = 30) compared to a control group (n = 62). Exploratory analyses with various types of interaction partners and social settings revealed noticeable group differences in how the types were associated with SSA (e.g. acquaintance, close friend/romantic partner) and how they influenced the effect of judgmentalness and unfamiliarity on SSA (e.g. authority, work/school). Overall, the findings highlight the role of contextual associations in social anxiety, and the benefits and the need for more comprehensive approaches with EMA in studying social anxiety, particularly its contextual aspects. (shrink)