BACKGROUND: Limited evidence exists regarding long-term effectiveness and safety of aldosterone antagonists (AAs) versus beta blockers (BBs) as fourth-line antihypertensive agents in patients with resistant hypertension (RH). We evaluated the comparative effectiveness and safety of aldosterone AA versus BB. METHODS: We conducted a real-world retrospective cohort study using IBM MarketScan commercial claims and Medicare Supplemental claims (2007–2019). Patients with RH entered the cohort (ie, index date) when they newly initiated either AA or BB. The effectiveness outcome was major adverse cardiovascular events. Safety outcomes were hyperkalemia, gynecomastia, and kidney function deterioration. Potential confounding was addressed by adjustment for baseline characteristics via stabilized inverse probability of treatment weighting (SIPTW) based on propensity scores. Cox proportional hazards regression with SIPTWs were used to estimate adjusted hazard ratio (aHR) and 95% CI comparing risk for outcomes between AA and BB groups. RESULTS: We identified 80 598 patients with RH (mean age: 61 years, 51% males), of which 6626 initiated AA and 73 972 initiated BB as the fourth antihypertensive agent. Among patients with RH, initiation of AA as a fourth-line antihypertensive agent did not significantly reduce major adverse cardiovascular event risk relative to BB initiation (aHR, 0.77 [95% CI, 0.50–1.19]) but did substantially increase the risk of hyperkalemia (aHR, 3.86 [95% CI, 2.78–5.34]), gynecomastia (aHR, 9.51 [95% CI, 5.69–15.89]), and kidney function deterioration (aHR, 1.63 [95% CI, 1.34–1.99]). CONCLUSIONS: Long-term clinical trials powered to assess major adverse cardiovascular events are necessary to understand the risk-benefit trade-off of AA as fourth-line therapy for RH.