diff --git a/PyInstaller/__main__.py b/PyInstaller/__main__.py
index c096b60954..b8fc0f3493 100644
--- a/PyInstaller/__main__.py
+++ b/PyInstaller/__main__.py
@@ -197,6 +197,15 @@ def run(pyi_args: list | None = None, pyi_config: dict | None = None):
             )
             spec_file = args.filenames[0]
         else:
+            # Ensure that the given script files exist, before trying to generate the .spec file.
+            # This prevents us from overwriting an existing (and customized) .spec file if user makes a typo in the
+            # .spec file's suffix when trying to  build it, for example, `pyinstaller program.cpes` (see #8276).
+            # It also prevents creation of a .spec file when `pyinstaller program.py` is accidentally ran from a
+            # directory that does not contain the script (for example, due to failing to change the directory prior
+            # to running the command).
+            for filename in args.filenames:
+                if not os.path.isfile(filename):
+                    raise SystemExit(f"Script file {filename!r} does not exist.")
             spec_file = run_makespec(**vars(args))
 
         sys.argv = [spec_file, *spec_args]
diff --git a/news/8279.bugfix.rst b/news/8279.bugfix.rst
new file mode 100644
index 0000000000..193afe3baf
--- /dev/null
+++ b/news/8279.bugfix.rst
@@ -0,0 +1,5 @@
+When trying to run ``pyinstaller`` (or equivalent ``python -m PyInstaller``)
+against non-existing script file(s), exit immediately - without trying
+to write the .spec file and building it. This prevents us from overwriting
+an existing (and customized) .spec file if user makes a typo in the .spec
+file's suffix when trying to build it, for example, ``pyinstaller program.cpes``.