diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 09c43822e11e4..c87c2dc534f36 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -1254,7 +1254,7 @@ def to_string(
         col_space: int | list[int] | dict[Hashable, int] | None = None,
         header: bool | SequenceNotStr[str] = True,
         index: bool = True,
-        na_rep: str = "NaN",
+        na_rep: str = lib.no_default,
         formatters: fmt.FormattersType | None = None,
         float_format: fmt.FloatFormatType | None = None,
         sparsify: bool | None = None,
diff --git a/pandas/core/series.py b/pandas/core/series.py
index b02cee3c1fcf3..8746546611624 100644
--- a/pandas/core/series.py
+++ b/pandas/core/series.py
@@ -1733,7 +1733,7 @@ def to_string(
     def to_string(
         self,
         buf: FilePath | WriteBuffer[str] | None = None,
-        na_rep: str = "NaN",
+        na_rep: str = lib.no_default,
         float_format: str | None = None,
         header: bool = True,
         index: bool = True,
diff --git a/pandas/io/formats/format.py b/pandas/io/formats/format.py
index 1a7e4d7a80e13..7d23b111ddd67 100644
--- a/pandas/io/formats/format.py
+++ b/pandas/io/formats/format.py
@@ -1223,17 +1223,30 @@ def _format_strings(self) -> list[str]:
 
         def _format(x):
             if self.na_rep is not None and is_scalar(x) and isna(x):
-                if x is None:
-                    return "None"
-                elif x is NA:
-                    return str(NA)
-                elif lib.is_float(x) and np.isinf(x):
-                    # TODO(3.0): this will be unreachable when use_inf_as_na
-                    #  deprecation is enforced
-                    return str(x)
-                elif x is NaT or isinstance(x, (np.datetime64, np.timedelta64)):
-                    return "NaT"
-                return self.na_rep
+                if self.na_rep is lib.no_default:
+                    if x is None:
+                        return "None"
+                    elif x is NA:
+                        return str(NA)
+                    elif lib.is_float(x) and np.isinf(x):
+                        # TODO(3.0): this will be unreachable when use_inf_as_na
+                        #  deprecation is enforced
+                        return str(x)
+                    elif x is NaT or isinstance(x, (np.datetime64, np.timedelta64)):
+                        return "NaT"
+                    return "NaN"
+                else:
+                    if x is None:
+                        return "None"
+                    elif x is NA:
+                        return self.na_rep
+                    elif lib.is_float(x) and np.isinf(x):
+                        # TODO(3.0): this will be unreachable when use_inf_as_na
+                        #  deprecation is enforced
+                        return str(x)
+                    elif x is NaT or isinstance(x, (np.datetime64, np.timedelta64)):
+                        return "NaT"
+                    return self.na_rep
             elif isinstance(x, PandasObject):
                 return str(x)
             elif isinstance(x, StringDtype):