Purpose: Due to substantial increase in obesity, the demand for total knee arthroplasty (TKA) in obese and morbidly obese patients is higher than ever. This review aims to investigate midto long-term complications, revision rates, and outcome for morbidly obese, compared with non-obese after TKA. Methods: A systematic search was conducted in May 2021. Included studies reported revision rates for morbidly obese and non-obese with a mean follow-up of at least 2 years. Reported knee society score (KSS) has been used to compare the functional outcome. PRISMA protocol was followed, and PROSPERO registered (ID: CRD42021254119). Results: From 12 studies that met the inclusion criteria, a total of 1031 cases of morbidly obese and 9797 cases of non-obese controls were included. The risk ratio for revision was 1.48 for the morbidly obese, compared with non-obese (95% CI: 0.98 to 2.24; P= 0.06). Regarding aseptic and septic revision, the risk ratio was 1.44 (95% CI: 0.64 to 3.25; P= 0.37) and 2.22 (95% CI: 0.89 to 5.57; P= 0.09), respectively. The morbidly obese scored lower in Objective Knee Society Score (OKSS) and Functional Knee Society Score (FKSS) both preoperatively and postoperatively, compared with the non-obese; however, the two groups improved equally in function scores OKSS (P= 0.967) and FKSS (P= 0.834). Overall risk ratio for complications was 1.56 (95% CI: 0.98 to 2.48; P= 0.06). Conclusions: The gained benefit in functional outcome surpasses the increase in risk of revision and complications for the morbidly obese in TKA surgery.