BACKGROUND: Fewer than 20% of eligible patients with heart failure with reduced ejection fraction receive all 4 pillars of guideline-directed medical therapy. Understanding disparities by race, ethnicity, sex, and adverse social determinants of health is necessary to equitably optimize quadruple therapy. METHODS: Utilizing the American Heart Association’s Get With The Guidelines-Heart Failure registry, we examined associations between race and ethnicity, sex, and adverse social determinants of health (insurance type and documented social need [any barrier to accessing health care]) with quadruple therapy optimization (QTO) in patients with heart failure with reduced ejection fraction hospitalized between July 1, 2021, and September 30, 2023, with complete medication data at discharge. We calculated adjusted mean differences (AMDs) in the discharge QTO score (range, 0%–100%) reflecting the proportion of eligible use of renin-angiotensin system inhibitors, β-blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter-2 inhibitors and compared across demographic and adverse social determinants of health groups. RESULTS: Among 82 637 patients (median age, 66 years; 32.5% female; 57.0% non-Hispanic White; 76.4% prior heart failure with reduced ejection fraction), the overall mean QTO score was 56.2% (SD, 25.5). After adjustment, compared with non-Hispanic White individuals, Black (AMD, 2.56 percentage points [95% CI, 2.16–2.96]) and Hispanic individuals (AMD, 0.71 percentage points [95% CI, 0.11–1.31]) had higher QTO scores. Females had higher QTO scores than males (AMD, 1.94 percentage points [95% CI, 1.58–2.31]). Patients with no insurance (AMD, −4.90 percentage points [−5.62 to −4.17]), Medicaid (AMD, −0.45 percentage points [−0.89 to −0.01]), and Medicare (AMD, −1.64 percentage points [−2.10 to −1.18]) had lower QTO scores versus private insurance. Those with an identified social need (n=24 651) had lower QTO scores than those without (AMD, −3.40 percentage points [95% CI, −4.10 to −2.71]). CONCLUSIONS: Disparities in QTO were most evident for patients with no insurance, Medicaid, Medicare, or potentially an identified social need. Future efforts should focus on reducing gaps to improve equitable guideline-directed medical therapy use.