diff --git a/tests/modules/programs/difftastic/default.nix b/tests/modules/programs/difftastic/default.nix
new file mode 100644
index 000000000..e85dd67d6
--- /dev/null
+++ b/tests/modules/programs/difftastic/default.nix
@@ -0,0 +1,7 @@
+{
+  difftastic-basic = ./difftastic-basic.nix;
+  difftastic-with-git-integration = ./difftastic-with-git-integration.nix;
+  difftastic-with-git-external-diff = ./difftastic-with-git-external-diff.nix;
+  difftastic-with-git-difftool = ./difftastic-with-git-difftool.nix;
+  difftastic-migration = ./difftastic-migration.nix;
+}
diff --git a/tests/modules/programs/difftastic/difftastic-basic.nix b/tests/modules/programs/difftastic/difftastic-basic.nix
new file mode 100644
index 000000000..96f701874
--- /dev/null
+++ b/tests/modules/programs/difftastic/difftastic-basic.nix
@@ -0,0 +1,18 @@
+{
+  programs.difftastic = {
+    enable = true;
+    options = {
+      color = "always";
+      display = "side-by-side";
+    };
+  };
+  programs.git.enable = true;
+
+  nmt.script = ''
+    # Git config should NOT contain difftastic configuration since git.enable is false by default
+    assertFileExists home-files/.config/git/config
+    assertFileNotRegex home-files/.config/git/config 'external = .*/difft'
+    assertFileNotRegex home-files/.config/git/config 'tool = "difftastic"'
+    assertFileNotRegex home-files/.config/git/config '\[difftool "difftastic"\]'
+  '';
+}
diff --git a/tests/modules/programs/difftastic/difftastic-migration.nix b/tests/modules/programs/difftastic/difftastic-migration.nix
new file mode 100644
index 000000000..a51c19601
--- /dev/null
+++ b/tests/modules/programs/difftastic/difftastic-migration.nix
@@ -0,0 +1,31 @@
+{
+  lib,
+  options,
+  ...
+}:
+
+{
+  programs.git = {
+    enable = true;
+    difftastic = {
+      enable = true;
+      options = {
+        color = "always";
+        display = "side-by-side";
+      };
+    };
+  };
+
+  test.asserts.warnings.expected = [
+    "The option `programs.git.difftastic.options' defined in ${lib.showFiles options.programs.git.difftastic.options.files} has been renamed to `programs.difftastic.options'."
+    "The option `programs.git.difftastic.enable' defined in ${lib.showFiles options.programs.git.difftastic.enable.files} has been renamed to `programs.difftastic.enable'."
+    "`programs.difftastic.git.enable` automatic enablement is deprecated. Please explicitly set `programs.difftastic.git.enable = true`."
+  ];
+
+  nmt.script = ''
+    # Git config should contain difftastic configuration (backward compatibility)
+    assertFileExists home-files/.config/git/config
+    assertFileContains home-files/.config/git/config '[diff]'
+    assertFileRegex home-files/.config/git/config 'external = .*/difft.*--color.*--display'
+  '';
+}
diff --git a/tests/modules/programs/difftastic/difftastic-with-git-difftool.nix b/tests/modules/programs/difftastic/difftastic-with-git-difftool.nix
new file mode 100644
index 000000000..ac6c7ec9c
--- /dev/null
+++ b/tests/modules/programs/difftastic/difftastic-with-git-difftool.nix
@@ -0,0 +1,25 @@
+{
+  programs.difftastic = {
+    enable = true;
+    git = {
+      enable = true;
+      diffToolMode = true;
+    };
+    options = {
+      color = "always";
+      display = "side-by-side";
+    };
+  };
+
+  programs.git.enable = true;
+
+  nmt.script = ''
+    assertFileExists home-files/.config/git/config
+    assertFileContains home-files/.config/git/config '[diff]'
+    # Should have BOTH diff.external AND difftool config when diffToolMode is true
+    assertFileRegex home-files/.config/git/config 'external = .*/difft.*--color.*--display'
+    assertFileRegex home-files/.config/git/config 'tool = "difftastic"'
+    assertFileContains home-files/.config/git/config '[difftool "difftastic"]'
+    assertFileRegex home-files/.config/git/config 'cmd = .*/difft.*--color.*--display.*\$LOCAL \$REMOTE'
+  '';
+}
diff --git a/tests/modules/programs/difftastic/difftastic-with-git-external-diff.nix b/tests/modules/programs/difftastic/difftastic-with-git-external-diff.nix
new file mode 100644
index 000000000..82c872e0a
--- /dev/null
+++ b/tests/modules/programs/difftastic/difftastic-with-git-external-diff.nix
@@ -0,0 +1,25 @@
+{
+  programs.difftastic = {
+    enable = true;
+    git = {
+      enable = true;
+      diffToolMode = false;
+    };
+    options = {
+      color = "always";
+      display = "side-by-side";
+    };
+  };
+
+  programs.git.enable = true;
+
+  nmt.script = ''
+    assertFileExists home-files/.config/git/config
+    assertFileContains home-files/.config/git/config '[diff]'
+    # Should have diff.external set
+    assertFileRegex home-files/.config/git/config 'external = .*/difft.*--color.*--display'
+    # Should NOT have difftool config when diffToolMode is explicitly false
+    assertFileNotRegex home-files/.config/git/config 'tool = "difftastic"'
+    assertFileNotRegex home-files/.config/git/config '\[difftool "difftastic"\]'
+  '';
+}
diff --git a/tests/modules/programs/difftastic/difftastic-with-git-integration.nix b/tests/modules/programs/difftastic/difftastic-with-git-integration.nix
new file mode 100644
index 000000000..1d05636b2
--- /dev/null
+++ b/tests/modules/programs/difftastic/difftastic-with-git-integration.nix
@@ -0,0 +1,22 @@
+{
+  programs.difftastic = {
+    enable = true;
+    git.enable = true;
+    options = {
+      color = "always";
+      display = "side-by-side";
+    };
+  };
+
+  programs.git.enable = true;
+
+  nmt.script = ''
+    assertFileExists home-files/.config/git/config
+    assertFileContains home-files/.config/git/config '[diff]'
+    # Should have diff.external set
+    assertFileRegex home-files/.config/git/config 'external = .*/difft.*--color.*--display'
+    # Should NOT have difftool config since diffToolMode is false
+    assertFileNotRegex home-files/.config/git/config 'tool = "difftastic"'
+    assertFileNotRegex home-files/.config/git/config '\[difftool "difftastic"\]'
+  '';
+}