  The topological $\theta$-angle is central to the understanding of a plethora
of phenomena in condensed matter and high-energy physics such as the strong CP
problem, dynamical quantum topological phase transitions, and the
confinement--deconfinement transition. Difficulties arise when probing the
effects of the topological $\theta$-angle using classical methods, in
particular through the appearance of a sign problem in numerical simulations.
Quantum simulators offer a powerful alternate venue for realizing the
$\theta$-angle, which has hitherto remained an outstanding challenge due to the
difficulty of introducing a dynamical electric field in the experiment. Here,
we report on the experimental realization of a tunable topological
$\theta$-angle in a Bose--Hubbard gauge-theory quantum simulator, implemented
through a tilted superlattice potential that induces an effective background
electric field. We demonstrate the rich physics due to this angle by the direct
observation of the confinement--deconfinement transition of $(1+1)$-dimensional
quantum electrodynamics. Using an atomic-precision quantum gas microscope, we
distinguish between the confined and deconfined phases by monitoring the
real-time evolution of particle--antiparticle pairs, which exhibit constrained
(ballistic) propagation for a finite (vanishing) deviation of the
$\theta$-angle from $\pi$. Our work provides a major step forward in the
realization of topological terms on modern quantum simulators, and the
exploration of rich physics they have been theorized to entail.
