  We report the first discovery of a transiting circumbinary planet detected
from a single sector of TESS data. During Sector 21, the planet TIC 172900988b
transited the primary star and then 5 days later it transited the secondary
star. The binary is itself eclipsing, with a period of P = 19.7 days and an
eccentricity of 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 M1 = 1.2384 +/- 0.0007 MSun and R1 = 1.3827 +/- 0.0016 RSun
for the primary and M2 = 1.2019 +/- 0.0007 MSun and R2 = 1.3124 +/- 0.0012 RSun
for the secondary. The radius of the planet is R3 = 11.25 +/- 0.44 REarth
(1.004 +/- 0.039 RJup). 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 < M3 < 981
MEarth (2.65 < M3 < 3.09 MJup), 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 a V = 10.141 mag, the system is accessible for high-resolution
spectroscopic observations, e.g. Rossiter-McLaughlin effect and transit
spectroscopy.
