diff --git a/plutus-core/changelog.d/20240520_192738_effectfully_remove_UnknownBuiltin.md b/plutus-core/changelog.d/20240520_192738_effectfully_remove_UnknownBuiltin.md
new file mode 100644
index 00000000000..5fe160f9919
--- /dev/null
+++ b/plutus-core/changelog.d/20240520_192738_effectfully_remove_UnknownBuiltin.md
@@ -0,0 +1,3 @@
+### Removed
+
+- `UnknownBuiltin` and `UnknownBuiltinType` in #6064.
diff --git a/plutus-core/plutus-core/src/PlutusCore/Error.hs b/plutus-core/plutus-core/src/PlutusCore/Error.hs
index 7b4b1e8d42a..7ec30398602 100644
--- a/plutus-core/plutus-core/src/PlutusCore/Error.hs
+++ b/plutus-core/plutus-core/src/PlutusCore/Error.hs
@@ -55,8 +55,7 @@ throwingEither r e = case e of
 
 -- | An error encountered during parsing.
 data ParserError
-    = UnknownBuiltinType !T.Text !SourcePos
-    | BuiltinTypeNotAStar !T.Text !SourcePos
+    = BuiltinTypeNotAStar !T.Text !SourcePos
     | UnknownBuiltinFunction !T.Text !SourcePos ![T.Text]
     | InvalidBuiltinConstant !T.Text !T.Text !SourcePos
     deriving stock (Eq, Ord, Generic)
@@ -171,8 +170,6 @@ instance Pretty SourcePos where
     pretty = pretty . sourcePosPretty
 
 instance Pretty ParserError where
-    pretty (UnknownBuiltinType s loc)       =
-        "Unknown built-in type" <+> squotes (pretty s) <+> "at" <+> pretty loc
     pretty (BuiltinTypeNotAStar ty loc)     =
         "Expected a type of kind star (to later parse a constant), but got:" <+>
             squotes (pretty ty) <+> "at" <+> pretty loc
diff --git a/plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/Exception.hs b/plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/Exception.hs
index 2ffd78de907..354750cc82c 100644
--- a/plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/Exception.hs
+++ b/plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/Exception.hs
@@ -60,7 +60,6 @@ data MachineError fun
       -- ^ A builtin expected a term argument, but something else was received
     | UnexpectedBuiltinTermArgumentMachineError
       -- ^ A builtin received a term argument when something else was expected
-    | UnknownBuiltin fun
     | NonConstrScrutinized
     | MissingCaseBranch Word64
     deriving stock (Show, Eq, Functor, Generic)
@@ -139,8 +138,6 @@ instance (HasPrettyDefaults config ~ 'True, Pretty fun) =>
         "A builtin received a term argument when something else was expected"
     prettyBy _      (UnliftingMachineError unliftingError)  =
         pretty unliftingError
-    prettyBy _      (UnknownBuiltin fun)                  =
-        "Encountered an unknown built-in function:" <+> pretty fun
     prettyBy _      NonConstrScrutinized =
         "A non-constructor value was scrutinized in a case expression"
     prettyBy _      (MissingCaseBranch i) =