Background and Objectives: Total knee arthroplasty (TKA) is an effective treatment option for severe knee osteoarthritis. Understanding the impact of diabetes mellitus (DM) on postoperative outcomes is crucial for improving patient satisfaction after TKA. This study aimed to investigate the influence of DM on postoperative complications and mortality after TKA. Materials and Methods: We conducted a systematic review and meta-analysis by searching relevant studies published before December 2023 in the PubMed, EMBASE, Cochrane Library, Medline, and Web of Science databases. The assessment included demographic data, comorbidities, and postoperative complications after primary TKA for both DM and non-DM patients. The odds ratio (OR) was used to represent the estimate of risk of a specific outcome. Results: Thirty-nine studies were finally included in this meta-analysis. Patients with DM had higher rates of periprosthetic joint infection (OR: 1.71, 95% confidence interval [CI]: 1.46–2.00, p < 0.01) and prosthesis revision (OR: 1.37, 95% CI: 1.23–1.52, p < 0.01). Moreover, patients with DM showed an elevated incidence of pneumonia (OR: 1.54, 95% CI: 1.15–2.07, p < 0.01), urinary tract infection (OR: 1.86, 95% CI: 1.07–3.26, p = 0.02), and sepsis (OR: 1.61, 95% CI: 1.46–1.78, p < 0.01). Additionally, the postoperative risk of cardiovascular (OR: 2.49, 95% CI: 1.50–4.17, p < 0.01) and cerebrovascular (OR: 2.38, 95% CI: 1.48–3.81, p < 0.01) events was notably higher in patients with DM. The presence of DM increased the risk of deep vein thrombosis (OR: 1.58, 95% CI: 1.22–2.04, p < 0.01), but did not lead to an increased risk of pulmonary embolism. Most importantly, DM was associated with a higher mortality rate within 30 days after TKA (OR: 1.27, 95% CI: 1.02–1.60, p = 0.03). Conclusions: Patients with DM exhibited a higher rate of postoperative complications after TKA, and DM was associated with a higher mortality rate within 30 days after TKA. It is crucial to educate patients about the perioperative risk and develop evidence-based guidelines to prevent complications after TKA.