Aside from depression and anxiety, less is known about the relationship of problematic smartphone use (PSU) to other psychopathology-related variables. The authors' aim was to test previously neglected variables in relation to PSU: rumination and excessive reassurance seeking behavior (ERSB). The authors recruited 295 college students for a web-based survey of smartphone use frequency, PSU, depression and anxiety, ruminative thinking, and ERSB. The authors tested linear regression and mediation models, assessing rumination and ERSB as mediating associations between depression/anxiety severity with PSU, adjusting for age, sex, and smartphone use frequency. Results demonstrate that ERSB was significantly related to PSU severity, and ERSB mediated the association between rumination and PSU. Furthermore, the combination of rumination and ERSB mediated relations between both depression and anxiety severity with PSU. Results provide evidence for ERSB as an important variable in understanding relationships between psychopathology symptoms and PSU severity among college students.