  We present the first numerical study of gravitational waves (GWs) from
collapsar disks, using state-of-the-art 3D general-relativistic
magnetohydrodynamic simulations of collapsing stars. These simulations
incorporate a fixed Kerr metric for the central black hole (BH) and employ
simplified prescriptions for disk cooling. We find that cooled disks with an
expected scale height ratio of $H/R\gtrsim0.1$ at $\sim10$ gravitational radii
induce Rossby instability in compact, high-density rings. The trapped Rossby
vortices generate vigorous coherent emission regardless of disk magnetization
and BH spin. For BH mass of $\sim10\,M_\odot$, the GW spectrum peaks at
$\sim100\,{\rm Hz}$ with some breadth due to various non-axisymmetric modes.
The spectrum shifts toward lower frequencies as the disk viscously spreads and
the circularization radius of the infalling gas increases. Weaker-cooled disks
with $H/R\gtrsim0.3$ form a low-density extended structure of spiral arms,
resulting in a broader, lower-amplitude spectrum. Assuming an optimistic
detection threshold with a matched-filter SNR of 20 and a rate similar to
supernovae Ib/c, LIGO-Virgo-KAGRA (LVK) could detect $\lesssim1$ event
annually, suggesting that GW events may already be hidden in observed data.
Third-generation GW detectors could detect dozens to hundreds of collapsar
disks annually, depending on the cooling strength and the disk formation rate.
The GW amplitudes from collapsar disks are $\gtrsim100$ times higher with a
substantially greater event rate than those from core-collapse supernovae,
making them potentially the most promising burst-type GW class for LVK and
Cosmic Explorer. This highlights the importance of further exploration and
modeling of disk-powered GWs, promising insights into collapsing star physics.
