We report the discovery and characterization of NGTS-39 b (TIC 453147896 b), a warm Jupiter transiting a Sun-like star on a 58.2 d, eccentric (e = $0.386\pm 0.019$) orbit. NGTS-39 b was first identified from a Transiting Exoplanet Survey Satellite single transit event, and subsequently confirmed with NGTS photometry and radial velocity measurements from CORALIE and HARPS. The host star is a bright ($T_{\mathrm{mag}}$ = 11.02) F9 dwarf with effective temperature of Teff = $6053_{-30}^{+67}$ K. NGTS-39 b is a Jupiter-sized gas giant with a radius of $1.088\pm 0.012$ RJ and a mass of $1.467\pm 0.081$ MJ. Its equilibrium temperature is $519_{-5}^{+6}$ K, placing it between short-period hot Jupiters and cold, Jupiter-like giants. The high orbital eccentricity and intermediate equilibrium temperature of NGTS-39 b make it a valuable test case for formation and migration models, particularly in the poorly sampled regime of long-period gas giants. The RV data show a linear trend of $\dot{\gamma } = -17.75$ m s$^{-1}$ yr$^{-1}$, which indicates the presence of an outer companion. The discovery of NGTS-39 b contributes to the small but growing population of transiting warm Jupiters with P> 50 d orbiting bright stars.