Abstract We report the first discovery of a transiting circumbinary planet detected from a single sector of Transiting Exoplanet Survey Satellite (TESS) data. During Sector 21, the planet TIC 172900988b transited the primary star and then five days later it transited the secondary star. The binary is itself eclipsing, with a period P ≈ 19.7 days and an eccentricity e ≈ 0.45. Archival data from ASAS-SN, Evryscope, KELT, and SuperWASP reveal a prominent apsidal motion of the binary orbit, caused by the dynamical interactions between the binary and the planet. A comprehensive photodynamical analysis of the TESS, archival and follow-up data yields stellar masses and radii of M ₁ = 1.2384 ±0.0007 M _⊙ and R ₁ = 1.3827 ± 0.0016 R _⊙ for the primary and M ₂ = 1.2019 ± 0.0007 M _⊙ and R ₂ = 1.3124 ±0.0012 R _⊙ for the secondary. The radius of the planet is R ₃ = 11.25 ± 0.44 R _⊕ (1.004 ± 0.039R _Jup). The planet’s mass and orbital properties are not uniquely determined—there are six solutions with nearly equal likelihood. Specifically, we find that the planet’s mass is in the range of 824 ≲ M ₃ ≲ 981 M _⊕ (2.65 ≲ M ₃ ≲ 3.09M _Jup), its orbital period could be 188.8, 190.4, 194.0, 199.0, 200.4, or 204.1 days, and the eccentricity is between 0.02 and 0.09. At V = 10.141 mag, the system is accessible for high-resolution spectroscopic observations, e.g., the Rossiter–McLaughlin effect and transit spectroscopy.