diff --git a/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals/in.md b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals/in.md
new file mode 100644
index 0000000000..dad0374702
--- /dev/null
+++ b/crates/red_knot_python_semantic/resources/mdtest/narrow/conditionals/in.md
@@ -0,0 +1,80 @@
+# Narrowing for `in` conditionals
+
+## `in` for tuples
+
+```py
+def _(x: int):
+    if x in (1, 2, 3):
+        reveal_type(x)  # revealed: int
+    else:
+        reveal_type(x)  # revealed: int
+```
+
+```py
+def _(x: str):
+    if x in ("a", "b", "c"):
+        reveal_type(x)  # revealed: str
+    else:
+        reveal_type(x)  # revealed: str
+```
+
+```py
+from typing import Literal
+
+def _(x: Literal[1, 2, "a", "b", False, b"abc"]):
+    if x in (1,):
+        reveal_type(x)  # revealed: Literal[1]
+    elif x in (2, "a"):
+        reveal_type(x)  # revealed: Literal[2, "a"]
+    elif x in (b"abc",):
+        reveal_type(x)  # revealed: Literal[b"abc"]
+    elif x not in (3,):
+        reveal_type(x)  # revealed: Literal["b", False]
+    else:
+        reveal_type(x)  # revealed: Never
+```
+
+```py
+def _(x: Literal["a", "b", "c", 1]):
+    if x in ("a", "b", "c", 2):
+        reveal_type(x)  # revealed: Literal["a", "b", "c"]
+    else:
+        reveal_type(x)  # revealed: Literal[1]
+```
+
+## `in` for `str` and literal strings
+
+```py
+def _(x: str):
+    if x in "abc":
+        reveal_type(x)  # revealed: str
+    else:
+        reveal_type(x)  # revealed: str
+```
+
+```py
+from typing import Literal
+
+def _(x: Literal["a", "b", "c", "d"]):
+    if x in "abc":
+        reveal_type(x)  # revealed: Literal["a", "b", "c"]
+    else:
+        reveal_type(x)  # revealed: Literal["d"]
+```
+
+```py
+def _(x: Literal["a", "b", "c", "e"]):
+    if x in "abcd":
+        reveal_type(x)  # revealed: Literal["a", "b", "c"]
+    else:
+        reveal_type(x)  # revealed: Literal["e"]
+```
+
+```py
+def _(x: Literal[1, "a", "b", "c", "d"]):
+    # error: [unsupported-operator]
+    if x in "abc":
+        reveal_type(x)  # revealed: Literal["a", "b", "c"]
+    else:
+        reveal_type(x)  # revealed: Literal[1, "d"]
+```
diff --git a/crates/red_knot_python_semantic/src/types.rs b/crates/red_knot_python_semantic/src/types.rs
index d5ed2b6d77..48da634620 100644
--- a/crates/red_knot_python_semantic/src/types.rs
+++ b/crates/red_knot_python_semantic/src/types.rs
@@ -408,6 +408,11 @@ impl<'db> Type<'db> {
         matches!(self, Type::FunctionLiteral(..))
     }
 
+    pub fn is_union_of_single_valued(&self, db: &'db dyn Db) -> bool {
+        self.into_union()
+            .is_some_and(|union| union.elements(db).iter().all(|ty| ty.is_single_valued(db)))
+    }
+
     pub const fn into_int_literal(self) -> Option<i64> {
         match self {
             Type::IntLiteral(value) => Some(value),
@@ -422,6 +427,10 @@ impl<'db> Type<'db> {
         }
     }
 
+    pub fn is_string_literal(&self) -> bool {
+        matches!(self, Type::StringLiteral(..))
+    }
+
     #[track_caller]
     pub fn expect_int_literal(self) -> i64 {
         self.into_int_literal()
@@ -5403,6 +5412,14 @@ impl<'db> StringLiteralType<'db> {
     pub fn python_len(&self, db: &'db dyn Db) -> usize {
         self.value(db).chars().count()
     }
+
+    /// Return an iterator over each character in the string literal.
+    /// as would be returned by Python's `iter()`.
+    pub fn iter_each_char(&self, db: &'db dyn Db) -> impl Iterator<Item = Self> {
+        self.value(db)
+            .chars()
+            .map(|c| StringLiteralType::new(db, c.to_string().as_str()))
+    }
 }
 
 #[salsa::interned(debug)]
diff --git a/crates/red_knot_python_semantic/src/types/narrow.rs b/crates/red_knot_python_semantic/src/types/narrow.rs
index ccf1816db0..ceaec3d3c1 100644
--- a/crates/red_knot_python_semantic/src/types/narrow.rs
+++ b/crates/red_knot_python_semantic/src/types/narrow.rs
@@ -19,6 +19,8 @@ use rustc_hash::FxHashMap;
 use std::collections::hash_map::Entry;
 use std::sync::Arc;
 
+use super::UnionType;
+
 /// Return the type constraint that `test` (if true) would place on `definition`, if any.
 ///
 /// For example, if we have this code:
@@ -288,6 +290,28 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
         NarrowingConstraints::from_iter([(symbol, ty)])
     }
 
+    fn evaluate_expr_in(&mut self, lhs_ty: Type<'db>, rhs_ty: Type<'db>) -> Option<Type<'db>> {
+        if lhs_ty.is_single_valued(self.db) || lhs_ty.is_union_of_single_valued(self.db) {
+            match rhs_ty {
+                Type::Tuple(rhs_tuple) => Some(UnionType::from_elements(
+                    self.db,
+                    rhs_tuple.elements(self.db),
+                )),
+
+                Type::StringLiteral(string_literal) => Some(UnionType::from_elements(
+                    self.db,
+                    string_literal
+                        .iter_each_char(self.db)
+                        .map(Type::StringLiteral),
+                )),
+
+                _ => None,
+            }
+        } else {
+            None
+        }
+    }
+
     fn evaluate_expr_compare(
         &mut self,
         expr_compare: &ast::ExprCompare,
@@ -371,6 +395,16 @@ impl<'db> NarrowingConstraintsBuilder<'db> {
                         ast::CmpOp::Eq if lhs_ty.is_literal_string() => {
                             constraints.insert(symbol, rhs_ty);
                         }
+                        ast::CmpOp::In => {
+                            if let Some(ty) = self.evaluate_expr_in(lhs_ty, rhs_ty) {
+                                constraints.insert(symbol, ty);
+                            }
+                        }
+                        ast::CmpOp::NotIn => {
+                            if let Some(ty) = self.evaluate_expr_in(lhs_ty, rhs_ty) {
+                                constraints.insert(symbol, ty.negate(self.db));
+                            }
+                        }
                         _ => {
                             // TODO other comparison types
                         }