Guidelines suggest thromboprophylaxis for ambulatory cancer patients starting chemotherapy with an intermediate to high risk of venous thromboembolism (VTE) according to Khorana score. Data on thromboprophylaxis efficacy in different Khorana score risk groups remain ambiguous. We sought to evaluate thromboprophylaxis in patients with an intermediate- to high-risk ($2 points) Khorana score and an intermediate-risk score (2 points) or high-risk score ($3 points) separately. MEDLINE, Embase, and CENTRAL were searched for randomized controlled trials (RCTs) comparing thromboprophylaxis with placebo or standard care in ambulatory cancer patients. Outcomes were VTE, major bleeding, and all-cause mortality. Relative risks (RRs) were calculated in a profile-likelihood random-effects model. Six RCTs were identified, involving 4626 cancer patients. Thromboprophylaxis with direct oral anticoagulants (DOACs) or low molecular weight heparin (LMWH) significantly reduced VTE risk in intermediate- to high-risk (RR, 0.51; 95% confidence interval [CI], 0.34-0.67), intermediate-risk (RR, 0.58; 95% CI, 0.36-0.83), and high-risk patients (RR, 0.45; 95% CI, 0.28-0.67); the numbers needed to treat (NNTs) were 25 (intermediate to high risk), 34 (intermediate risk), and 17 (high risk), respectively. There was no significant difference in major bleeding (RR, 1.06; 95% CI, 0.69-1.67) or all-cause mortality (RR, 0.90; 95% CI, 0.82-1.01). The numbers needed to harm (NNHs) for major bleeding in intermediate- to high-risk, intermediate-risk, and high-risk patients were 1000, 2500, and 334, respectively. The overall NNH was lower in DOAC studies (100) versus LMWH studies (2500). These findings indicate thromboprophylaxis effectively reduces the risk of VTE in patients with an intermediate- to high-risk Khorana score, although the NNT is twice as high for intermediate-risk patients compared with high-risk patients.