diff --git a/changelog.d/620.changed.md b/changelog.d/620.changed.md
new file mode 100644
index 000000000..902b25d78
--- /dev/null
+++ b/changelog.d/620.changed.md
@@ -0,0 +1 @@
+Update the ACA take-up parameter with a 2023-2026 time series built from CMS Marketplace open enrollment data and KFF eligibility estimates.
diff --git a/policyengine_us_data/parameters/take_up/aca.yaml b/policyengine_us_data/parameters/take_up/aca.yaml
index 98f920142..b1a40a7bb 100644
--- a/policyengine_us_data/parameters/take_up/aca.yaml
+++ b/policyengine_us_data/parameters/take_up/aca.yaml
@@ -4,7 +4,32 @@ metadata:
   unit: /1
   period: year
   reference:
-    - title: KFF "A Closer Look at the Remaining Uninsured Population Eligible for Medicaid and CHIP"
-      href: https://www.kff.org/uninsured/issue-brief/a-closer-look-at-the-remaining-uninsured-population-eligible-for-medicaid-and-chip/#:~:text=the%20uninsured%20rate%20dropped%20to,States%20began%20the
+    - title: KFF "Distribution of Eligibility for ACA Health Coverage Among the Remaining Uninsured"
+      href: https://www.kff.org/affordable-care-act/state-indicator/distribution-of-eligibility-for-aca-coverage-among-the-remaining-uninsured/
+    - title: CMS "2023 Marketplace Open Enrollment Period Public Use Files"
+      href: https://www.cms.gov/data-research/statistics-trends-reports/marketplace-products/2023-marketplace-open-enrollment-period-public-use-files
+    - title: CMS "2024 Marketplace Open Enrollment Period Public Use Files"
+      href: https://www.cms.gov/data-research/statistics-trends-reports/marketplace-products/2024-marketplace-open-enrollment-period-public-use-files
+    - title: CMS "2025 Marketplace Open Enrollment Period Public Use Files"
+      href: https://www.cms.gov/data-research/statistics-trends-reports/marketplace-products/2025-marketplace-open-enrollment-period-public-use-files
+    - title: CMS "2026 Marketplace Open Enrollment Period Public Use Files"
+      href: https://www.cms.gov/data-research/statistics-trends-reports/marketplace-products/2026-marketplace-open-enrollment-period-public-use-files
 values:
   2018-01-01: 0.672
+  # OEP APTC plan selections / (OEP APTC plan selections + tax-credit-eligible uninsured).
+  # Numerator: APTC plan selections summed from the CMS state-level OEP public
+  # use files: 14,797,921 (2023), 19,743,689 (2024), 22,380,137 (2025), and
+  # 20,035,756 (2026). Plan selections are used instead of effectuated
+  # enrollment so all four years come from the same source; effectuated
+  # enrollment for 2026 is not yet published.
+  # Denominator addend: KFF reports 8,163,000 uninsured people eligible for
+  # Marketplace tax credits, published for 2023 only. The same count is reused
+  # for 2024-2026, which likely understates take-up in those years because
+  # later enrollment growth draws people out of that uninsured pool.
+  # 2021-2022 stay on the 2018 baseline because the KFF denominator reflects
+  # the 2023 uninsured population (post-Medicaid-unwinding) and does not
+  # backcast to earlier years.
+  2023-01-01: 0.644483
+  2024-01-01: 0.707489
+  2025-01-01: 0.732739
+  2026-01-01: 0.710519
diff --git a/tests/unit/test_stochastic_variables.py b/tests/unit/test_stochastic_variables.py
index a168f3ee3..3c1ca127b 100644
--- a/tests/unit/test_stochastic_variables.py
+++ b/tests/unit/test_stochastic_variables.py
@@ -37,6 +37,13 @@ def test_aca_rate_loads(self):
         rate = load_take_up_rate("aca", 2022)
         assert 0 < rate <= 1
 
+    def test_aca_rate_uses_marketplace_oep_enrollment_time_series(self):
+        assert load_take_up_rate("aca", 2022) == 0.672
+        assert np.isclose(load_take_up_rate("aca", 2023), 0.644483)
+        assert np.isclose(load_take_up_rate("aca", 2024), 0.707489)
+        assert np.isclose(load_take_up_rate("aca", 2025), 0.732739)
+        assert np.isclose(load_take_up_rate("aca", 2026), 0.710519)
+
     def test_head_start_rate_loads(self):
         rate = load_take_up_rate("head_start", 2022)
         assert 0 < rate <= 1