Clustering of chronic conditions is associated with high healthcare costs. Sustaining blood pressure (BP) control could be a strategy to prevent high-cost multimorbidity clusters. To determine the association between sustained systolic BP (SBP) control and incident multimorbidity cluster dyads and triads. Cohort study of Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial (ALLHAT) linked to Medicare claims. ALLHAT included adults with hypertension and ≥1 coronary heart disease risk factor. This analysis was restricted to 5234 participants with ≥ 8 SBP measurements during a 48-month BP assessment period. SBP control was defined as <140 mm Hg at <50%, 50 to <75%, 75 to <100%, and 100% of study visits during the BP assessment period. High-cost multimorbidity clusters included dyads (stroke/chronic kidney disease [CKD], stroke/chronic obstructive pulmonary disease [COPD], stroke/heart failure [HF], stroke/asthma, COPD/CKD) and triads (stroke/CKD/asthma, stroke/CKD/COPD, stroke/CKD/depression, stroke/CKD/HF, stroke/HF/asthma) identified during follow-up. Incident dyads occurred in 1334 (26%) participants and triads occurred in 481 (9%) participants over a median follow-up of 9.2 years. Among participants with SBP control at <50%, 50 to <75%, 75 to <100%, and 100% of visits, 32%, 23%, 23%, and 19% of participants developed high-cost dyads, respectively, and 13%, 9%, 8%, and 5% of participants developed high-cost triads, respectively. Compared to those with sustained BP control at <50% of visits, adjusted HRs (95% CI) for incident dyads were 0.66 (0.57, 0.75), 0.67 (0.59, 0.77), and 0.51 (0.42, 0.62) for SBP control at 50 to <75%, 75 to <100%, and 100% of visits, respectively. The corresponding HRs (95% CI) for incident triads were 0.69 (0.55, 0.85), 0.56 (0.44, 0.71), and 0.32 (0.22, 0.47). Among Medicare beneficiaries in ALLHAT, sustained SBP was associated with a lower risk of developing high-cost multimorbidity dyads and triads.