  Fine-structure dynamics in solar prominences holds critical clues to
understanding their physical nature of significant space-weather implications.
We report evidence of rotational motions of horizontal helical threads in two
active-region prominences observed by the \emph{Hinode} and/or \emph{IRIS}
satellites at high resolution. In the first event, we found transverse motions
of brightening threads at speeds up to 55~km~s$^{-1}$ seen in the plane of the
sky. Such motions appeared as sinusoidal space--time trajectories with a
typical period of $\sim$390~s, which is consistent with plane-of-sky
projections of rotational motions. Phase delays at different locations suggest
propagation of twists along the threads at phase speeds of 90--270~km~s$^{-1}$.
At least 15 episodes of such motions occurred in two days, none associated with
any eruption. For these episodes, the plane-of-sky speed is linearly correlated
with the vertical travel distance, suggestive of a constant angular speed. In
the second event, we found Doppler velocities of 30--40~km~s$^{-1}$ in opposite
directions in the top and bottom portions of the prominence, comparable to the
plane-of-sky speed. The moving threads have about twice broader line widths
than stationary threads. These observations, when taken together, provide
strong evidence for rotations of helical prominence threads, which were likely
driven by unwinding twists triggered by magnetic reconnection between twisted
prominence magnetic fields and ambient coronal fields.
