This systematic review evaluates the diagnostic and theranostic utilities of fibroblast activation protein inhibitor (FAPI PET) in patients with soft tissue sarcomas (STS). PubMed, Scopus, and Web of Science were researched for clinical studies employing FAPI PET in STS, covering literature up to October 15, 2024. This review synthesized 36 studies, involving 316 patients and about 30 STS subtypes. Twenty-one case reports highlighted FAPI's diagnostic utility. Comparative analyses often favored FAPI PET/CT over FDG PET/CT for tumor delineation, showing higher SUVmax and tumor-to-background ratios, especially in vascular tumors. Seven original studies consistently reported reliable FAPI uptake metrics across STS subtypes in 253 patients, strongly correlating with FAP immunohistochemistry. FAPI PET/CT generally demonstrated higher uptake, improved accuracy, and better lesion detection than FDG PET/CT, supporting its use in staging and restaging of recurrent and metastatic STS. Eight studies assessed FAPI PET/CT for FAPI-based radioligand therapy (RLT) eligibility, with posttherapeutic outcome predominantly resulted in disease stabilization (16 out of 36 patients), and a favorable safety profile. FAPI PET/CT outperforms FDG in detecting recurrent/metastatic STS, offering higher accuracy in most STS types. FAPI-based radioligand therapy is a promising, safe treatment for challenging STS, with encouraging efficacy.