The prognostic significance of the systemic immune-inflammation index (SII) in breast cancer is unknown. Here, we aimed to explore the connection between pretreatment SII and the survival of patients with triple-negative breast cancer (TNBC). We enrolled 160 TNBC patients treated in our hospital between May 2000 and June 2012. We employed the Kaplan-Meier curve and log-rank test to assess overall survival (OS), disease-free survival (DFS), and distant metastasis-free survival (DMFS). We identified the prognostic significance of SII using the Cox regression model. The Kaplan-Meier curve revealed the median OS as 44.2 and 82.4 months in high and low SII TNBC patients, respectively (<0.001). According to univariate and multivariate analyses, increased SII correlated with poor OS (HR =2.91, 95% CI: 2.00-4.23, <0.001; HR =2.60, 95% CI: 1.74-3.88, <0.001). The DFS and DMFS of patients with high SII were 18.8 and 23.8 months, respectively, while those of patients with low SII were 29 and 45.2 months, respectively, (<0.001). Further univariate analyses showed a significant correlation between SII and DFS and DMFS (<0.01), while results from multivariate analyses suggested that SII is an independent prognostic factor for DFS (=0.045), but not for DMFS (=0.078). The area under the receiver operating characteristics curves for SII to differentiate between long and short OS, DFS, and DMFS were 0.69, 0.60, and 0.64, respectively. Our findings may point to SII having an independent prognostic significance in TNBC patients. Prospective in-depth studies, using a larger sample size, are required to further investigate the precise role of SII in TNBC before clinical use.