  NASA's Transiting Exoplanet Survey Satellite (TESS) has identified over 7,000
candidate exoplanets via the transit method, with gas giants among the most
readily detected due to their large radii. Even so, long intervals between TESS
observations for much of the sky lead to candidates for which only a single
transit is detected in one TESS sector, leaving those candidate exoplanets with
unconstrained orbital periods. Here, we confirm the planetary nature of TIC
393818343 b, originally identified via a single TESS transit, using radial
velocity data and ground-based photometric observations from citizen scientists
with the Unistellar Network and Exoplanet Watch. We determine a period of $P$ =
16.24921 $\substack{+0.00010 \\ -0.00011}$ days, a mass $M_{P}$ = 4.34 $\pm$
0.15 $M_{J}$, and semi-major axis $a$ = 0.1291 $\substack{+0.0021 \\ -0.0022}$
au, placing TIC 393818343 b in the "warm Jupiter" population of exoplanets.
With an eccentricity $e$ = 0.6058 $\pm$ 0.0023, TIC 393818343 b is the most
eccentric warm Jupiter to be discovered by TESS orbiting less than 0.15 au from
its host star and therefore an excellent candidate for follow-up, as it may
inform our future understanding of how hot and warm Jupiter populations are
linked.
