diff --git a/src/Init/Data/String/Slice.lean b/src/Init/Data/String/Slice.lean
index 3f7f23e4b3..e3a5f80870 100644
--- a/src/Init/Data/String/Slice.lean
+++ b/src/Init/Data/String/Slice.lean
@@ -1249,7 +1249,8 @@ def foldr {α : Type u} (f : Char → α → α) (init : α) (s : Slice) : α :=
 Checks whether the slice can be interpreted as the decimal representation of a natural number.
 
 A slice can be interpreted as a decimal natural number if it is not empty and all the characters in
-it are digits.
+it are digits. Underscores ({lit}`_`) are allowed as digit separators for readability, but cannot appear
+at the start, at the end, or consecutively.
 
 Use {name (scope := "Init.Data.String.Slice")}`toNat?` or
 {name (scope := "Init.Data.String.Slice")}`toNat!` to convert such a slice to a natural number.
@@ -1260,21 +1261,39 @@ Examples:
  * {lean}`"5".toSlice.isNat = true`
  * {lean}`"05".toSlice.isNat = true`
  * {lean}`"587".toSlice.isNat = true`
+ * {lean}`"1_000".toSlice.isNat = true`
+ * {lean}`"100_000_000".toSlice.isNat = true`
  * {lean}`"-587".toSlice.isNat = false`
  * {lean}`" 5".toSlice.isNat = false`
  * {lean}`"2+3".toSlice.isNat = false`
  * {lean}`"0xff".toSlice.isNat = false`
+ * {lean}`"_123".toSlice.isNat = false`
+ * {lean}`"123_".toSlice.isNat = false`
+ * {lean}`"12__34".toSlice.isNat = false`
 -/
 @[inline]
 def isNat (s : Slice) : Bool :=
-  !s.isEmpty && s.all Char.isDigit
+  if s.isEmpty then
+    false
+  else
+    -- Track: isFirst, lastWasUnderscore, lastCharWasDigit, valid
+    let result := s.foldl (fun (isFirst, lastWasUnderscore, _lastCharWasDigit, valid) c =>
+      let isDigit := c.isDigit
+      let isUnderscore := c = '_'
+      let newValid := valid && (isDigit || isUnderscore) &&
+                      !(isFirst && isUnderscore) &&  -- Cannot start with underscore
+                      !(lastWasUnderscore && isUnderscore)  -- No consecutive underscores
+      (false, isUnderscore, isDigit, newValid))
+      (true, false, false, true)
+    -- Must be valid and last character must have been a digit (not underscore)
+    result.2.2.2 && result.2.2.1
 
 /--
 Interprets a slice as the decimal representation of a natural number, returning it. Returns
 {name}`none` if the slice does not contain a decimal natural number.
 
 A slice can be interpreted as a decimal natural number if it is not empty and all the characters in
-it are digits.
+it are digits. Underscores ({lit}`_`) are allowed as digit separators and are ignored during parsing.
 
 Use {name}`isNat` to check whether {name}`toNat?` would return {name}`some`.
 {name (scope := "Init.Data.String.Slice")}`toNat!` is an alternative that panics instead of
@@ -1285,6 +1304,8 @@ Examples:
  * {lean}`"0".toSlice.toNat? = some 0`
  * {lean}`"5".toSlice.toNat? = some 5`
  * {lean}`"587".toSlice.toNat? = some 587`
+ * {lean}`"1_000".toSlice.toNat? = some 1000`
+ * {lean}`"100_000_000".toSlice.toNat? = some 100000000`
  * {lean}`"-587".toSlice.toNat? = none`
  * {lean}`" 5".toSlice.toNat? = none`
  * {lean}`"2+3".toSlice.toNat? = none`
@@ -1292,7 +1313,7 @@ Examples:
 -/
 def toNat? (s : Slice) : Option Nat :=
   if s.isNat then
-    some <| s.foldl (fun n c => n * 10 + (c.toNat - '0'.toNat)) 0
+    some <| s.foldl (fun n c => if c = '_' then n else n * 10 + (c.toNat - '0'.toNat)) 0
   else
     none
 
@@ -1301,7 +1322,7 @@ Interprets a slice as the decimal representation of a natural number, returning
 slice does not contain a decimal natural number.
 
 A slice can be interpreted as a decimal natural number if it is not empty and all the characters in
-it are digits.
+it are digits. Underscores ({lit}`_`) are allowed as digit separators and are ignored during parsing.
 
 Use {name}`isNat` to check whether {name}`toNat!` would return a value. {name}`toNat?` is a safer
 alternative that returns {name}`none` instead of panicking when the string is not a natural number.
@@ -1310,10 +1331,11 @@ Examples:
  * {lean}`"0".toSlice.toNat! = 0`
  * {lean}`"5".toSlice.toNat! = 5`
  * {lean}`"587".toSlice.toNat! = 587`
+ * {lean}`"1_000".toSlice.toNat! = 1000`
 -/
 def toNat! (s : Slice) : Nat :=
   if s.isNat then
-    s.foldl (fun n c => n * 10 + (c.toNat - '0'.toNat)) 0
+    s.foldl (fun n c => if c = '_' then n else n * 10 + (c.toNat - '0'.toNat)) 0
   else
     panic! "Nat expected"
 
diff --git a/src/Init/Data/String/Substring.lean b/src/Init/Data/String/Substring.lean
index d9007f7013..0f7df7c0b2 100644
--- a/src/Init/Data/String/Substring.lean
+++ b/src/Init/Data/String/Substring.lean
@@ -403,25 +403,39 @@ Examples:
 Checks whether the substring can be interpreted as the decimal representation of a natural number.
 
 A substring can be interpreted as a decimal natural number if it is not empty and all the characters
-in it are digits.
+in it are digits. Underscores ({lit}`_`) are allowed as digit separators for readability, but cannot appear
+at the start, at the end, or consecutively.
 
 Use `Substring.toNat?` to convert such a substring to a natural number.
 -/
 @[inline] def isNat (s : Substring.Raw) : Bool :=
-  !s.isEmpty && s.all fun c => c.isDigit
+  if s.isEmpty then
+    false
+  else
+    -- Track: isFirst, lastWasUnderscore, lastCharWasDigit, valid
+    let result := s.foldl (fun (isFirst, lastWasUnderscore, _lastCharWasDigit, valid) c =>
+      let isDigit := c.isDigit
+      let isUnderscore := c = '_'
+      let newValid := valid && (isDigit || isUnderscore) &&
+                      !(isFirst && isUnderscore) &&  -- Cannot start with underscore
+                      !(lastWasUnderscore && isUnderscore)  -- No consecutive underscores
+      (false, isUnderscore, isDigit, newValid))
+      (true, false, false, true)
+    -- Must be valid and last character must have been a digit (not underscore)
+    result.2.2.2 && result.2.2.1
 
 /--
 Checks whether the substring can be interpreted as the decimal representation of a natural number,
 returning the number if it can.
 
 A substring can be interpreted as a decimal natural number if it is not empty and all the characters
-in it are digits.
+in it are digits. Underscores ({lit}`_`) are allowed as digit separators and are ignored during parsing.
 
 Use `Substring.isNat` to check whether the substring is such a substring.
 -/
 def toNat? (s : Substring.Raw) : Option Nat :=
   if s.isNat then
-    some <| s.foldl (fun n c => n*10 + (c.toNat - '0'.toNat)) 0
+    some <| s.foldl (fun n c => if c = '_' then n else n*10 + (c.toNat - '0'.toNat)) 0
   else
     none
 
diff --git a/tests/lean/run/string_toNat_underscores.lean b/tests/lean/run/string_toNat_underscores.lean
new file mode 100644
index 0000000000..23d6e6d867
--- /dev/null
+++ b/tests/lean/run/string_toNat_underscores.lean
@@ -0,0 +1,88 @@
+/-!
+Tests for `String.toNat?` and `String.toInt?` with underscore support.
+
+This ensures that numeric parsing functions accept the same underscore digit separators
+that are allowed in Lean numeric literals, for consistency.
+
+See: https://github.com/leanprover/lean4/issues/11538
+-/
+
+-- Basic underscore tests for String.Slice.isNat
+#guard "1_000".toSlice.isNat = true
+#guard "100_000".toSlice.isNat = true
+#guard "1_000_000".toSlice.isNat = true
+#guard "100_000_000".toSlice.isNat = true
+
+-- Edge cases for isNat
+#guard "_123".toSlice.isNat = false  -- Cannot start with underscore
+#guard "123_".toSlice.isNat = false  -- Cannot end with underscore
+#guard "12__34".toSlice.isNat = false  -- Cannot have consecutive underscores
+#guard "1_2_3".toSlice.isNat = true  -- Single underscores are fine
+
+-- toNat? basic tests
+#guard "1_000".toSlice.toNat? = some 1000
+#guard "100_000".toSlice.toNat? = some 100000
+#guard "1_000_000".toSlice.toNat? = some 1000000
+#guard "100_000_000".toSlice.toNat? = some 100000000
+
+-- More complex patterns
+#guard "1_2_3_4_5".toSlice.toNat? = some 12345
+#guard "9_99".toSlice.toNat? = some 999
+#guard "5_0_0_0".toSlice.toNat? = some 5000
+
+-- Edge cases that should fail
+#guard "_123".toSlice.toNat? = none
+#guard "123_".toSlice.toNat? = none
+#guard "12__34".toSlice.toNat? = none
+#guard "12_3_".toSlice.toNat? = none
+#guard "_".toSlice.toNat? = none
+
+-- Verify numeric values are correct
+#guard "1_234".toSlice.toNat? = "1234".toSlice.toNat?
+#guard "987_654_321".toSlice.toNat? = "987654321".toSlice.toNat?
+
+-- Tests for String.toNat? (not just Slice)
+#guard "1_000".toNat? = some 1000
+#guard "100_000".toNat? = some 100000
+#guard "1_000_000".toNat? = some 1000000
+#guard "_123".toNat? = none
+#guard "123_".toNat? = none
+
+-- Tests for String.Slice.isInt
+#guard "-1_000".toSlice.isInt = true
+#guard "-100_000".toSlice.isInt = true
+#guard "1_000".toSlice.isInt = true  -- Positive numbers are also ints
+
+-- Edge cases for isInt
+#guard "-_123".toSlice.isInt = false  -- Cannot have underscore right after minus
+#guard "-123_".toSlice.isInt = false  -- Cannot end with underscore
+#guard "-12__34".toSlice.isInt = false  -- Cannot have consecutive underscores
+
+-- toInt? basic tests
+#guard "-1_000".toSlice.toInt? = some (-1000)
+#guard "-100_000".toSlice.toInt? = some (-100000)
+#guard "-1_000_000".toSlice.toInt? = some (-1000000)
+#guard "1_000".toSlice.toInt? = some 1000
+#guard "100_000".toSlice.toInt? = some 100000
+
+-- Edge cases for toInt?
+#guard "-_123".toSlice.toInt? = none
+#guard "-123_".toSlice.toInt? = none
+#guard "-12__34".toSlice.toInt? = none
+
+-- Verify numeric values are correct
+#guard "-1_234".toSlice.toInt? = "-1234".toSlice.toInt?
+#guard "987_654_321".toSlice.toInt? = "987654321".toSlice.toInt?
+
+-- Tests for String.toInt? (not just Slice)
+#guard "-1_000".toInt? = some (-1000)
+#guard "1_000".toInt? = some 1000
+#guard "-_123".toInt? = none
+#guard "-123_".toInt? = none
+
+-- Compatibility with existing behavior (no underscores)
+#guard "0".toNat? = some 0
+#guard "123".toNat? = some 123
+#guard "".toNat? = none
+#guard "-456".toInt? = some (-456)
+#guard "789".toInt? = some 789