diff --git a/specparam/modes/definitions.py b/specparam/modes/definitions.py
index bdb5c8c4..8961d636 100644
--- a/specparam/modes/definitions.py
+++ b/specparam/modes/definitions.py
@@ -79,6 +79,28 @@
     powers_space='log10',
 )
 
+## AP - Knee with Constant Mode
+
+params_knee_constant = ParamDefinition(OrderedDict({
+    'offset' : 'Offset of the aperiodic component.',
+    'exponent' : 'Exponent of the aperiodic component.',
+    'knee' : 'Knee of the aperiodic component.',
+    'constant' : 'Constant value which the aperiodic component decays to, after the exponent.',
+}))
+
+ap_knee_constant = Mode(
+    name='knee_constant',
+    component='aperiodic',
+    description='Fit a Lorentzian function that decays to a constant.',
+    formula=r'XX',
+    func=knee_constant_function,
+    jacobian=None,
+    params=params_knee_constant,
+    ndim=1,
+    freq_space='linear',
+    powers_space='log10',
+)
+
 
 # Collect available aperiodic modes
 AP_MODES = {
diff --git a/specparam/modes/funcs.py b/specparam/modes/funcs.py
index 1d8bd078..dba4aac4 100644
--- a/specparam/modes/funcs.py
+++ b/specparam/modes/funcs.py
@@ -344,6 +344,31 @@ def double_expo_function(xs, *params):
     return ys
 
 
+def knee_constant_function(xs, *params):
+    """Knee function with a constant, for fitting aperiodic component.
+
+    Parameters
+    ----------
+    xs : 1d array
+        Input x-axis values.
+    *params : float
+        Parameters (offset, exp, knee, constant) that define the function.
+
+    Returns
+    -------
+    ys : 1d array
+        Output values for the fit function.
+    """
+
+    ys = np.zeros_like(xs)
+
+    offset, exp, knee, constant = params
+
+    ys = ys + offset - np.log10(1 / (knee**(exp) + xs**(exp)) + constant)
+
+    return ys
+
+
 def linear_function(xs, *params):
     r"""Linear fitting function.