diff --git a/fooof/synth.py b/fooof/synth.py
index 15a87b681..ea5b38b11 100644
--- a/fooof/synth.py
+++ b/fooof/synth.py
@@ -375,6 +375,51 @@ def gen_power_vals(xs, background_params, gauss_params, nlv):
 
     return ys
 
+def rotate_spectrum(freqs, power_spectrum, delta_f, f_rotation):
+    """Rotate a power spectrum about a frequency point, changing the power law exponent.
+
+    Parameters
+    ----------
+    freqs : 1d array
+        Frequency axis of input power spectrum, in Hz.
+    power_spectrum : 1d array
+        Power values of the spectrum that is to be rotated.
+    delta_f : float
+        Change in power law exponent to be applied.
+            Positive is counterclockwise rotation (flatten)
+            Negative is clockwise rotation (steepen).
+    f_rotation : float
+        Frequency, in Hz, at which to do the rotation, such that power at that frequency is unchanged.
+
+    Returns
+    -------
+    1d array
+        Rotated psd.
+
+    """
+
+    # Check that the requested frequency rotation value is within the given range
+    if f_rotation < freqs.min() or f_rotation > freqs.max():
+        raise ValueError('Rotation frequency not within frequency range.')
+
+    f_mask = np.zeros_like(freqs)
+
+
+    # NOTE: Update this to use data checking from fooof.fit
+    if freqs[0] == 0.:
+        # If starting freq is 0Hz, default power at 0Hz to old value because log
+        #   will return inf. Use case occurs in simulating/manipulating time series.
+        f_mask[0] = 1.
+        f_mask[1:] = 10**(np.log10(np.abs(freqs[1:])) * (delta_f))
+    else:
+        # Otherwise, apply rotation to all frequencies.
+        f_mask = 10**(np.log10(np.abs(freqs)) * (delta_f))
+
+    f_mask = f_mask / f_mask[np.where(freqs >= f_rotation)[0][0]]
+
+    return f_mask * power_spectrum
+
+
 ###################################################################################################
 ###################################################################################################
 
diff --git a/fooof/tests/test_synth.py b/fooof/tests/test_synth.py
index 00553f040..03ef03a93 100644
--- a/fooof/tests/test_synth.py
+++ b/fooof/tests/test_synth.py
@@ -55,6 +55,8 @@ def test_stepper():
 
     assert Stepper(8,12,.1)
 
+    # TODO: add more tests of Stepper
+
 
 def test_param_sampler():
 
@@ -136,6 +138,20 @@ def test_gen_power_values():
 
     assert np.all(ys)
 
+def test_rotate_spectrum():
+
+    # Create a spectrum to use for test rotations
+    freqs, spectrum = gen_power_spectrum([1, 100], [1, 1], [])
+
+    # Check that rotation transforms the power spectrum
+    rotated_spectrum = rotate_spectrum(freqs, spectrum, delta_f=0.5, f_rotation=25.)
+    assert not np.all(rotated_spectrum == spectrum)
+
+    # Check that 0 rotation returns the same spectrum
+    rotated_spectrum = rotate_spectrum(freqs, spectrum, delta_f=0., f_rotation=25.)
+    assert np.all(rotated_spectrum == spectrum)
+
+
 def test_check_iter():
 
     # Note: generator case not tested