diff --git a/changes/3975-arsenron.md b/changes/3975-arsenron.md
new file mode 100644
index 0000000000..7aaea271cb
--- /dev/null
+++ b/changes/3975-arsenron.md
@@ -0,0 +1 @@
+Remove undefined behaviour when `env_prefix` had characters in common with `env_nested_delimiter`
diff --git a/pydantic/env_settings.py b/pydantic/env_settings.py
index 35b96970f6..6c2d1b3ffd 100644
--- a/pydantic/env_settings.py
+++ b/pydantic/env_settings.py
@@ -63,6 +63,7 @@ def _build_values(
             env_nested_delimiter=(
                 _env_nested_delimiter if _env_nested_delimiter is not None else self.__config__.env_nested_delimiter
             ),
+            env_prefix_len=len(self.__config__.env_prefix),
         )
         file_secret_settings = SecretsSettingsSource(secrets_dir=_secrets_dir or self.__config__.secrets_dir)
         # Provide a hook to set built-in sources priority and add / remove sources
@@ -142,14 +143,19 @@ def __repr__(self) -> str:
 
 
 class EnvSettingsSource:
-    __slots__ = ('env_file', 'env_file_encoding', 'env_nested_delimiter')
+    __slots__ = ('env_file', 'env_file_encoding', 'env_nested_delimiter', 'env_prefix_len')
 
     def __init__(
-        self, env_file: Optional[StrPath], env_file_encoding: Optional[str], env_nested_delimiter: Optional[str] = None
+        self,
+        env_file: Optional[StrPath],
+        env_file_encoding: Optional[str],
+        env_nested_delimiter: Optional[str] = None,
+        env_prefix_len: int = 0,
     ):
         self.env_file: Optional[StrPath] = env_file
         self.env_file_encoding: Optional[str] = env_file_encoding
         self.env_nested_delimiter: Optional[str] = env_nested_delimiter
+        self.env_prefix_len: int = env_prefix_len
 
     def __call__(self, settings: BaseSettings) -> Dict[str, Any]:  # noqa C901
         """
@@ -228,7 +234,9 @@ def explode_env_vars(self, field: ModelField, env_vars: Mapping[str, Optional[st
         for env_name, env_val in env_vars.items():
             if not any(env_name.startswith(prefix) for prefix in prefixes):
                 continue
-            _, *keys, last_key = env_name.split(self.env_nested_delimiter)
+            # we remove the prefix before splitting in case the prefix has characters in common with the delimiter
+            env_name_without_prefix = env_name[self.env_prefix_len :]
+            _, *keys, last_key = env_name_without_prefix.split(self.env_nested_delimiter)
             env_var = result
             for key in keys:
                 env_var = env_var.setdefault(key, {})
diff --git a/tests/test_settings.py b/tests/test_settings.py
index c11cdbb739..018cc5a49c 100644
--- a/tests/test_settings.py
+++ b/tests/test_settings.py
@@ -134,6 +134,33 @@ class Config:
     }
 
 
+def test_nested_env_delimiter_with_prefix(env):
+    class Subsettings(BaseSettings):
+        banana: str
+
+    class Settings(BaseSettings):
+        subsettings: Subsettings
+
+        class Config:
+            env_nested_delimiter = '_'
+            env_prefix = 'myprefix_'
+
+    env.set('myprefix_subsettings_banana', 'banana')
+    s = Settings()
+    assert s.subsettings.banana == 'banana'
+
+    class Settings(BaseSettings):
+        subsettings: Subsettings
+
+        class Config:
+            env_nested_delimiter = '_'
+            env_prefix = 'myprefix__'
+
+    env.set('myprefix__subsettings_banana', 'banana')
+    s = Settings()
+    assert s.subsettings.banana == 'banana'
+
+
 def test_nested_env_delimiter_complex_required(env):
     class Cfg(BaseSettings):
         v: str = 'default'