This communication proposes a method for rapid in-field estimation of the 3-D array factor (AF) of active phased array antennas (APAAs). The proposed system integrates a coupling probe into each antenna element and employs an equally-distributed combiner and electronic phase sweeping to reconstruct the in-field array factor (IAF) across hemispherical space without requiring far-field conditions or mechanical rotation. A mathematical model is developed to analyze the relationship between the AF and the IAF, including the impact of mutual coupling. To validate the proposed concept, an X-band 8 × 4 APAA is designed and implemented with the coupling probe structure. Simulations confirm that the IAF closely follows the AF behavior in both ideal and beam-shaped conditions, with correlation coefficients exceeding 0.86. In experiments conducted at 8.15 GHz, the measured IAF captures the mainlobe shape, sidelobe suppression, and beam distortions under various scenarios. Correlation values between IAFs and conventionally measured array patterns (APs) were found to be greater than 0.87 in all cases. These results demonstrate that the proposed method provides a compact and scalable solution for monitoring the APAA’s performance.