We present the discovery of the single-transit planet candidate TOI 429b. TOI 429 was observed by TESS for four consecutive sectors but only showed one transit in November 2018, consistent with a 0.8 ± 0.06 RJ planet. A multi-year RV campaign, including PFS, HARPS, CORALIE, FEROS, and CHIRON data, was able to determine the period of TOI 429b to be 339 ± 3 days and its mass of 0.9 ± 0.06 MJ. The host star has Tmag = 10.1, Teff=5216±125 K, R*=0.95±0.05RSun and M*=0.89±0.11 MSun, placing TOI 429b in the habitable zone of a bright nearby star with a high probability of re-transiting during the TESS extended mission. TOI 429b is the longest-period planet discovered by TESS to date, and represents a rare find in terms of follow-up possibilities. In particular, atmospheric studies of TOI 429b will enable comparisons with the atmospheres of the giant planets in our own solar system, and with known hot and warm Jupiters. The discovery also sets the stage for a search of additional planets in the system.