  We report galaxy MACS0416-Y3 behind the lensing cluster MACSJ0416.1--2403 as
a tentative rotating disk at $z=8.34$ detected through its [OIII]$\lambda5007$
emission in JWST NIRCam wide-field slitless spectroscopic observations. The
discovery is based on our new grism dynamical modeling methodology for JWST
NIRCam slitless spectroscopy, using the data from ``Median-band Astrophysics
with the Grism of NIRCam in Frontier Fields'' (MAGNIF), a JWST Cycle-2 program.
The [OIII]$\lambda5007$ emission line morphology in grism data shows velocity
offsets compared to the F480M direct imaging, suggestive of rotation. Assuming
a geometrically thin disk model, we constrain the rotation velocity of $v_{\rm
rot}=58^{+53}_{-35}$ km s$^{-1}$ via forward modeling of the two-dimensional
(2D) spectrum. We obtain the kinematic ratio of $v_{\rm
rot}/\sigma_v=1.6^{+1.9}_{-0.9}$, where $\sigma_v$ is the velocity dispersion,
in line with a quasi-stable thin disk. The resulting dynamical mass is
estimated to be $\log(M_{\rm dyn}/M_{\odot})=8.4^{+0.5}_{-0.7}$. If the
rotation confirmed, our discovery suggests that rotating gaseous disks may have
already existed within 600 million years after Big Bang.
