Theia is an ESA astrometry mission designed to achieve a precision of 0.3 µas, enabling ultra-precise measurements of stellar positions. When focused on giant planet detection, Theia can carry out a large-scale census of Jupiter-like planets orbiting stars at 150 parsecs and beyond, including systems that are inaccessible to radial velocity due to high stellar activity or youth. This provides a valuable insight into the early dynamical evolution of planetary systems and offers important constraints on the formation and migration mechanisms of giant planets. Theia's astrometry complements radial velocity (RV) measurements by providing true planetary masses and full orbital inclinations. It can also reveal mutual inclinations in multi-planet systems, helping to map the three-dimensional architecture of planetary systems. Furthermore, its 0.33 deg² field enables the astrometric monitoring of stars up to ten magnitudes fainter than the primary targets, greatly increasing its statistical reach. Theia can also identify prime targets for direct imaging and atmospheric characterisation for future missions.