The aim of this systematic review was to explore the effectiveness of pelvic-floor muscle training (PFMT) in the treatment of women with pelvic organ prolapse (POP) who had undergone either surgery or only conservative treatment, based on a selection of randomized clinical trials (RCT). The search was carried out in PubMed, Cochrane, Scopus, CINAHL, and PEDro databases between April 2021 and October 2021 using the following MeSH terms or keywords: “pelvic organ prolapse”, “POP”, “pelvic floor muscle training”, “pelvic floor muscle exercise”, “kegel exercise”, and “surgery”. The methodological quality of the studies was assessed using the PEDro scale. Eighteen RCTs were included in this review. The findings showed improvements in symptoms associated with POP, in pelvic-floor function, and in quality of life in women who performed a PFMT protocol. However, PFMT did not produce significant changes in sexual function, and the results of the change in POP stage were inconclusive. When viewing PFMT as a complementary treatment to surgery, no significant improvements were observed in any of the analyzed variables. In conclusion, a PFMT program is an effective way to improve the pelvic, urinary, and intestinal symptoms associated with POP; function of the pelvic floor; and quality of life. PFMT as an adjunct to surgery does not seem to provide a greater benefit than surgical treatment alone. RCTs of higher methodological quality, with a larger sample size and a longer follow-up, are needed to confirm the results.