diff --git a/doc/releases/1.16.1dev.rst b/doc/releases/1.16.1dev.rst
index 9efeb5437..e038495ae 100644
--- a/doc/releases/1.16.1dev.rst
+++ b/doc/releases/1.16.1dev.rst
@@ -64,6 +64,11 @@ Instrument-specific Updates
         - Platescale updated from an order-dependent value, to being 0.164
           arcsec/pixel for all orders
 - Add new P200/DBSP reid_arxiv template for 1200/7100 with D55 dichroic
+- GMOS-South: PypeIt reductions of GMOS-South MOS observations now properly
+  account for tilted slits in the mask design file. Previously, these data
+  could only be reduced without including any information from the slitmask
+  design.
+
 
 Script Changes
 --------------
diff --git a/pypeit/spectrographs/gemini_gmos.py b/pypeit/spectrographs/gemini_gmos.py
index 3c861056a..955cf41d6 100644
--- a/pypeit/spectrographs/gemini_gmos.py
+++ b/pypeit/spectrographs/gemini_gmos.py
@@ -618,11 +618,8 @@ def get_slitmask(self, filename):
         offsets = np.sqrt(
                 mask_tbl['slitpos_x'].to('arcsec').value**2 +
                 mask_tbl['slitpos_y'].to('arcsec').value**2)
-        # NOT READY FOR TILTS
-        if np.any(np.invert(np.isclose(mask_tbl['slittilt'].value, 0.))):
-            msgs.error('NOT READY FOR TILTED SLITS')
-        # NOT SURE WE HAVE THE TILT SIGN CORRECT
-        slit_pas = posx_pa + mask_tbl['slittilt'].to('deg').value
+        # Sign here checked by Jack O'Donnell
+        slit_pas = posx_pa - mask_tbl['slittilt'].to('deg').value
         off_signs = np.ones_like(slit_pas)
         negy = mask_tbl['slitpos_y'] < 0.
         off_signs[negy] = -1.
@@ -695,7 +692,7 @@ def get_maskdef_slitedges(self, ccdnum=None, filename=None, debug=None,
         self.get_slitmask(maskfile)
 
         # Binning of flat
-        _, bin_spat= parse.parse_binning(binning)
+        _, bin_spat = parse.parse_binning(binning)
 
         # Slit center
         slit_coords = SkyCoord(ra=self.slitmask.onsky[:,0],
@@ -712,15 +709,16 @@ def get_maskdef_slitedges(self, ccdnum=None, filename=None, debug=None,
         left_edges = []
         right_edges = []
         for islit in range(self.slitmask.nslits):
-            # DEBUGGING
-            #islit = 14  # 10043
+            # JOD: Need to adjust predicted images by slit tilt (which is
+            # slit PA relative to mask PA)
+            slittilt = self.slitmask.onsky[islit,4] - self.slitmask.posx_pa
             # Left coord
             left_coord = slit_coords[islit].directional_offset_by(
                 self.slitmask.onsky[islit,4]*units.deg - 180.*units.deg,
-                self.slitmask.onsky[islit,2]*units.arcsec/2.)
+                self.slitmask.onsky[islit,2]*units.arcsec/2./np.cos(np.radians(slittilt)))
             right_coord = slit_coords[islit].directional_offset_by(
                 self.slitmask.onsky[islit,4]*units.deg,
-                self.slitmask.onsky[islit,2]*units.arcsec/2.)
+                self.slitmask.onsky[islit,2]*units.arcsec/2./np.cos(np.radians(slittilt)))
 
             got_it = False
             for kk, iwcs in enumerate(wcss):
@@ -748,6 +746,7 @@ def get_maskdef_slitedges(self, ccdnum=None, filename=None, debug=None,
         left_edges = np.array(left_edges).astype(float)
         right_edges = np.array(right_edges).astype(float)
         sortindx = np.argsort(left_edges)
+
         return left_edges, right_edges, sortindx, self.slitmask
 
 class GeminiGMOSSHamSpectrograph(GeminiGMOSSpectrograph):