diff --git a/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP032_2.py b/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP032_2.py
index 2987164454..9dbc7f385a 100644
--- a/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP032_2.py
+++ b/crates/ruff_linter/resources/test/fixtures/pyupgrade/UP032_2.py
@@ -26,3 +26,8 @@
 "{.real}".format({1, 2})
 "{.real}".format({1: 2, 3: 4})
 "{}".format((i for i in range(2)))
+
+# https://github.com/astral-sh/ruff/issues/21017
+"{.real}".format(1_2)
+"{0.real}".format(1_2)
+"{a.real}".format(a=1_2)
diff --git a/crates/ruff_linter/src/rules/pyupgrade/rules/f_strings.rs b/crates/ruff_linter/src/rules/pyupgrade/rules/f_strings.rs
index 46379c8ef0..f92372f43a 100644
--- a/crates/ruff_linter/src/rules/pyupgrade/rules/f_strings.rs
+++ b/crates/ruff_linter/src/rules/pyupgrade/rules/f_strings.rs
@@ -160,7 +160,11 @@ fn parenthesize(expr: &Expr, text: &str, context: FormatContext) -> bool {
                 value: ast::Number::Int(..),
                 ..
             }),
-        ) => text.chars().all(|c| c.is_ascii_digit()),
+        ) => text
+            .chars()
+            // Ignore digit separators so decimal literals like `1_2` still count as pure digits.
+            .filter(|c| *c != '_')
+            .all(|c| c.is_ascii_digit()),
         // E.g., `{x, y}` should be parenthesized in `f"{(x, y)}"`.
         (
             _,
diff --git a/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP032_2.py.snap b/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP032_2.py.snap
index f3324ea02b..835b1ea651 100644
--- a/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP032_2.py.snap
+++ b/crates/ruff_linter/src/rules/pyupgrade/snapshots/ruff_linter__rules__pyupgrade__tests__UP032_2.py.snap
@@ -385,6 +385,7 @@ help: Convert to f-string
 26 + f"{({1, 2}).real}"
 27 | "{.real}".format({1: 2, 3: 4})
 28 | "{}".format((i for i in range(2)))
+29 | 
 
 UP032 [*] Use f-string instead of `format` call
   --> UP032_2.py:27:1
@@ -402,6 +403,8 @@ help: Convert to f-string
    - "{.real}".format({1: 2, 3: 4})
 27 + f"{({1: 2, 3: 4}).real}"
 28 | "{}".format((i for i in range(2)))
+29 | 
+30 | # https://github.com/astral-sh/ruff/issues/21017
 
 UP032 [*] Use f-string instead of `format` call
   --> UP032_2.py:28:1
@@ -410,6 +413,8 @@ UP032 [*] Use f-string instead of `format` call
 27 | "{.real}".format({1: 2, 3: 4})
 28 | "{}".format((i for i in range(2)))
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+29 |
+30 | # https://github.com/astral-sh/ruff/issues/21017
    |
 help: Convert to f-string
 25 | "{.real}".format([1, 2])
@@ -417,3 +422,56 @@ help: Convert to f-string
 27 | "{.real}".format({1: 2, 3: 4})
    - "{}".format((i for i in range(2)))
 28 + f"{(i for i in range(2))}"
+29 | 
+30 | # https://github.com/astral-sh/ruff/issues/21017
+31 | "{.real}".format(1_2)
+
+UP032 [*] Use f-string instead of `format` call
+  --> UP032_2.py:31:1
+   |
+30 | # https://github.com/astral-sh/ruff/issues/21017
+31 | "{.real}".format(1_2)
+   | ^^^^^^^^^^^^^^^^^^^^^
+32 | "{0.real}".format(1_2)
+33 | "{a.real}".format(a=1_2)
+   |
+help: Convert to f-string
+28 | "{}".format((i for i in range(2)))
+29 | 
+30 | # https://github.com/astral-sh/ruff/issues/21017
+   - "{.real}".format(1_2)
+31 + f"{(1_2).real}"
+32 | "{0.real}".format(1_2)
+33 | "{a.real}".format(a=1_2)
+
+UP032 [*] Use f-string instead of `format` call
+  --> UP032_2.py:32:1
+   |
+30 | # https://github.com/astral-sh/ruff/issues/21017
+31 | "{.real}".format(1_2)
+32 | "{0.real}".format(1_2)
+   | ^^^^^^^^^^^^^^^^^^^^^^
+33 | "{a.real}".format(a=1_2)
+   |
+help: Convert to f-string
+29 | 
+30 | # https://github.com/astral-sh/ruff/issues/21017
+31 | "{.real}".format(1_2)
+   - "{0.real}".format(1_2)
+32 + f"{(1_2).real}"
+33 | "{a.real}".format(a=1_2)
+
+UP032 [*] Use f-string instead of `format` call
+  --> UP032_2.py:33:1
+   |
+31 | "{.real}".format(1_2)
+32 | "{0.real}".format(1_2)
+33 | "{a.real}".format(a=1_2)
+   | ^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+help: Convert to f-string
+30 | # https://github.com/astral-sh/ruff/issues/21017
+31 | "{.real}".format(1_2)
+32 | "{0.real}".format(1_2)
+   - "{a.real}".format(a=1_2)
+33 + f"{(1_2).real}"