  GRB 160821A is the third most energetic gamma ray burst observed by the {\it
Fermi} gamma-ray space telescope. Based on the observations made by Cadmium
Zinc Telluride Imager (CZTI) on board {\it AstroSat}, here we report the most
conclusive evidence to date of (i) high linear polarization ($66^{+26}_{-27}
\%$; $5.3 \sigma$ detection), and (ii) variation of its polarization angle with
time happening twice during the rise and decay phase of the burst at $3.5
\sigma$ and $3.1 \sigma$ detections respectively. All confidence levels are
reported for two parameters of interest. These observations strongly suggest
synchrotron radiation produced in magnetic field lines which are highly ordered
on angular scales of $1/\Gamma$, where $\Gamma $ is the Lorentz factor of the
outflow.
