Emotion regulation (ER) diversity, defined as the variety, frequency, and evenness of ER strategies used, may predict social anxiety severity. In a sample of individuals with high ( n = 113) and low ( n = 42) social anxiety severity, we tested whether four trait ER diversity metrics predicted group membership. We generalized existing trait ER diversity calculations to repeated measures data to test whether state-level metrics (using 2 weeks of ecological momentary assessment [EMA] data) predicted social anxiety severity within the higher severity group. As hypothesized, higher trait ER diversity within avoidance-oriented strategies predicted greater likelihood of belonging to the higher severity group. At the state level, higher diversity across all ER strategies, and within and between avoidance- and approach-oriented strategies, predicted higher social anxiety severity (but only after analyses controlled for number of submitted EMAs). Only diversity within avoidance-oriented strategies was significantly correlated across trait and state levels. Findings suggest that high avoidance-oriented ER diversity may co-occur with higher social anxiety severity.