Positron emission tomography (PET)–based multimodal imaging, including PET/computed tomography (PET/CT) and PET/magnetic resonance (PET/MR), plays an increasingly important role in precision radiotherapy by integrating metabolic and anatomical information. This review summarizes the clinical applications of PET‐guided imaging in tumor staging, target volume delineation, and radiotherapy planning. PET/CT remains the primary modality in most oncological settings owing to its geometric accuracy, rapid acquisition, and broad clinical availability, whereas PET/MR provides superior soft‐tissue contrast and is particularly valuable in neuro‐oncological and pelvic tumors. Recent advances in tumor‐specific radiotracers, image reconstruction, and artificial intelligence–assisted analysis have further improved the accuracy and efficiency of PET‐guided radiotherapy. Despite these advantages, challenges such as limited spatial resolution, variability in quantitative parameters, and high implementation costs persist. Emerging technologies, including total‐body PET, advanced detector systems, and AI‐driven adaptive radiotherapy, are expected to address these limitations and facilitate clinical translation. Overall, PET‐based multimodal imaging represents a critical component of biologically guided and personalized radiotherapy, with substantial potential to improve treatment precision and patient outcomes.