Secrecy outage probability is an important performance measure for physical layer security techniques. In this letter, analytical expressions for the secrecy outage probability in the presence of randomly moving interfering users and Nakagami- $m$ fading are derived. The derivation employs the random waypoint model for user mobility and the Gamma approximation for the aggregate interference. Numerical results are presented to quantify the impact of several important system parameters on the secrecy performance. They also show that the interference leads to a lower limit of the secrecy outage probability.