Triple-negative breast cancer (TNBC) is an aggressive subtype with a relatively poor prognosis, and its natural history without therapeutic intervention remains understudied. The purpose of this study was to investigate the natural history of untreated TNBC and construct a prognostic model utilizing data from the SEER database. Data from patients diagnosed with TNBC between 2010 and 2021 were analyzed. Median survival times for each tumor stage were calculated to estimate disease progression time (defined as the difference in median survival between consecutive stages). Cox regression was employed to identify independent prognostic factors in a training set, and a prognostic nomogram was developed based on these factors. Results showed that median survival times for untreated TNBC were 65 months (stage I), 28 months (stage II), 11 months (stage III), and 3 months (stage IV), with estimated progression times from stage I to II, II to III, and III to IV being 37 months, 17 months, and 8 months, respectively. Age, multifocal breast cancer, T stage, M stage, liver metastasis, and brain metastasis were identified as independent risk factors. The nomogram demonstrated excellent predictive performance, with a C-index of 0.795 in the training set and 0.759 in the validation set, supported by favorable calibration plots and decision curve analysis (DCA). In conclusion, untreated TNBC exhibited accelerated progression and deteriorating prognosis with increasing tumor stage, with age and tumor burden serving as key prognostic indicators, and the constructed nomogram provided a reliable tool for predicting survival in untreated TNBC patients.