diff --git a/changelog.d/ma-state-supplement-circuit-breaker.fixed.md b/changelog.d/ma-state-supplement-circuit-breaker.fixed.md
new file mode 100644
index 00000000..caca5a36
--- /dev/null
+++ b/changelog.d/ma-state-supplement-circuit-breaker.fixed.md
@@ -0,0 +1 @@
+Zero the imputed Massachusetts SSI state supplement so it does not inflate the Senior Circuit Breaker total income, matching TAXSIM (which does not model it).
diff --git a/policyengine_taxsim/core/input_mapper.py b/policyengine_taxsim/core/input_mapper.py
index 2be72a6b..cc34bb26 100644
--- a/policyengine_taxsim/core/input_mapper.py
+++ b/policyengine_taxsim/core/input_mapper.py
@@ -225,6 +225,17 @@ def form_household_situation(year, state, taxsim_vars):
             "commodity_supplemental_food_program"
         ] = {str(year): 0}
 
+    # Explicitly set the Massachusetts SSI state supplement to 0 so PolicyEngine
+    # does not impute it. TAXSIM does not model it, and Massachusetts counts cash
+    # public assistance (including SSI) in the Senior Circuit Breaker "total
+    # income" (MGL c.62 s.6(k)), so an imputed supplement understates the credit.
+    # See PolicyEngine/policyengine-taxsim#1031.
+    if state.upper() == "MA":
+        for person_name in household_situation["people"]:
+            household_situation["people"][person_name]["ma_state_supplement"] = {
+                str(year): 0
+            }
+
     # Explicitly set imputed medical expenses to 0 for all people. TAXSIM has no
     # medical-expense input, so PE's imputed Medicare Part B premiums would
     # otherwise flow through the federal itemized medical deduction into state
diff --git a/policyengine_taxsim/runners/policyengine_runner.py b/policyengine_taxsim/runners/policyengine_runner.py
index 9d24d103..e28b3313 100644
--- a/policyengine_taxsim/runners/policyengine_runner.py
+++ b/policyengine_taxsim/runners/policyengine_runner.py
@@ -159,6 +159,7 @@ def _initialize_dataset_structure(self) -> dict:
             "ak_energy_relief",
             "ak_permanent_fund_dividend",
             "ssi",  # SSI should be 0 to match TAXSIM (which doesn't model SSI)
+            "ma_state_supplement",  # MA SSI state supplement should be 0 to match TAXSIM (which doesn't model it); it feeds the MA Senior Circuit Breaker income
             "wic",  # WIC should be 0 to match TAXSIM (which doesn't model WIC)
             "snap",  # SNAP should be 0 to match TAXSIM (which doesn't model SNAP)
             "tanf",  # TANF should be 0 to match TAXSIM (which doesn't model TANF)
@@ -829,6 +830,9 @@ def generate(self) -> None:
                 data["ssi"][year_int] = np.zeros(
                     total_people_for_year
                 )  # Set SSI to 0 to match TAXSIM (which doesn't model SSI)
+                data["ma_state_supplement"][year_int] = np.zeros(
+                    total_people_for_year
+                )  # Set MA SSI state supplement to 0 to match TAXSIM; it feeds the MA Senior Circuit Breaker total income (MGL c.62 s.6(k)). See issue #1031.
                 data["wic"][year_int] = np.zeros(
                     total_people_for_year
                 )  # Set WIC to 0 to match TAXSIM (which doesn't model WIC)