  We present five simultaneous UV/X-ray observations of IC4329A by AstroSat,
performed over {a five-month} period. We utilize the excellent spatial
resolution of the Ultra-Violet Imaging Telescope (UVIT) onboard AstroSat to
reliably separate the intrinsic AGN flux from the host galaxy emission and to
correct for the Galactic and internal reddening, as well as the contribution
from the narrow and broad-line regions. We detect large-amplitude UV
variability, which is unusual for a large black hole mass AGN, like IC4329A,
over such a small period. In fact, the fractional variability amplitude is
larger in the UV band than in the X-ray band. This demonstrates that the
observed UV variability is intrinsic to the disk, and is not due to X-ray
illumination. The joint X-ray spectral analyses of five SXT and LAXPC spectral
data reveal a soft-X-ray excess component, a narrow iron-line (with no
indication of a significant Compton hump), and a steepening power-law
($\Delta\Gamma\sim 0.21$) with increasing X-ray flux. The soft excess component
could arise due to thermal Comptonization of the inner disk photons in a warm
corona with $kT_e\sim 0.26$ keV. The UV emission we detect acts as the primary
seed photons for the hot corona, which produces the broadband X-ray continuum.
The X-ray spectral variability is well described by the cooling of this corona
from $kT_e\sim42$ keV to $\sim 32$ keV with increasing UV flux, while the
optical depth remains constant at $\tau\sim 2.3$.
