This study aimed to (a) explore the stability of changes in mobile phone addiction and depressive symptoms across time and (b) clarify the direction of reciprocal longitudinal relationships between mobile phone addiction and depressive symptoms among Korean adolescents. We analyzed three-year longitudinal data from the Korean Children and Youth Panel Survey conducted by the National Youth Policy Institute in Korea. A total of 1877 valid responses from 2011 to 2013 were analyzed using autoregressive cross-lagged modeling. We found that each mobile phone addiction and depressive symptom in earlier years was associated with increasing severity in these conditions consistently over the three years. In addition, we found that the relationships between mobile phone addiction and depressive symptoms were bidirectional over the three years. The significant implications for these findings in the context of adolescent behavior are also discussed.