diff --git a/scikit_posthocs/_plotting.py b/scikit_posthocs/_plotting.py
index 20be9ca..1d6efa1 100644
--- a/scikit_posthocs/_plotting.py
+++ b/scikit_posthocs/_plotting.py
@@ -364,6 +364,7 @@ def critical_difference_diagram(
     ranks: Union[dict, Series],
     sig_matrix: DataFrame,
     *,
+    cd: Optional[float] = None,
     ax: Optional[Axes] = None,
     label_fmt_left: str = "{label} ({rank:.2g})",
     label_fmt_right: str = "({rank:.2g}) {label}",
@@ -412,6 +413,9 @@ def critical_difference_diagram(
         The object in which the plot will be built. Gets the current Axes
         by default (if None is passed).
 
+    cd : float, optional
+        Critical difference value to be plotted as a horizontal line above the axis.
+
     label_fmt_left : str, optional
         The format string to apply to the labels on the left side. The keywords
         label and rank can be used to specify the sample/estimator name and
@@ -588,6 +592,22 @@ def plot_items(points, xpos, label_fmt, color_palette, label_props):
             )
             ypos -= 1
 
+    def plot_cd(cd, label_props, crossbar_props):
+        """Plot CD horizontal line and label above OX axis if CD is passed."""
+        if cd is not None:
+            x_min, x_max = ax.get_xlim()
+            ax.axhline(0.5, 0, (cd / (x_max - x_min)), **crossbar_props)
+            ax.text((x_min + cd / 2), 0.65, f"CD = {cd:.2g}", **label_props)
+    
+    plot_cd(
+        cd,
+        crossbar_props=crossbar_props,
+        label_props={
+            "ha": "center",
+            **label_props,
+        }
+    )
+
     plot_items(
         points_left,
         xpos=points_left.iloc[0] - text_h_margin,