diff --git a/CHANGES.rst b/CHANGES.rst
index 7c13c942af..e0469340fc 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -111,6 +111,8 @@ Bug Fixes
   world and pixel (previously containing SkyCoord and pixel location tuples, respectively) are now
   each two separate columns for world_ra/world_dec and pixel_x/pixel_y, respectively. [#3089]
 
+- Aperture Photometry plugin no longer allows negative counts conversion factor. [#3154]
+
 Cubeviz
 ^^^^^^^
 
diff --git a/jdaviz/configs/cubeviz/plugins/tests/test_cubeviz_aperphot.py b/jdaviz/configs/cubeviz/plugins/tests/test_cubeviz_aperphot.py
index 032c9fa2ad..c600ee01c9 100644
--- a/jdaviz/configs/cubeviz/plugins/tests/test_cubeviz_aperphot.py
+++ b/jdaviz/configs/cubeviz/plugins/tests/test_cubeviz_aperphot.py
@@ -80,6 +80,11 @@ def test_cubeviz_aperphot_cube_orig_flux(cubeviz_helper, image_cube_hdu_obj_micr
     assert_allclose(row["mean"], 36 * flux_unit)
     assert np.isnan(row["slice_wave"])
 
+    # Invalid counts conversion factor
+    plg.counts_factor = -1
+    plg.vue_do_aper_phot()
+    assert "invalid counts" in plg.result_failed_msg
+
 
 def test_cubeviz_aperphot_generated_3d_gaussian_smooth(cubeviz_helper, image_cube_hdu_obj_microns):
     cubeviz_helper.load_data(image_cube_hdu_obj_microns, data_label="test")
diff --git a/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.py b/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.py
index bf9d63be8e..418bbadfef 100644
--- a/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.py
+++ b/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.py
@@ -637,7 +637,9 @@ def calculate_photometry(self, dataset=None, aperture=None, background=None,
                     include_pixarea_fac = True
             if img_unit != u.count:
                 try:
-                    ctfac = float(counts_factor if counts_factor is not None else self.counts_factor)  # noqa
+                    ctfac = float(counts_factor if counts_factor is not None else self.counts_factor)  # noqa: E501
+                    if ctfac < 0:
+                        raise ValueError('Counts conversion factor cannot be negative.')
                 except ValueError:  # Clearer error message
                     raise ValueError('Missing or invalid counts conversion factor')
                 if not np.allclose(ctfac, 0):
diff --git a/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.vue b/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.vue
index 0bdf42bbe1..7391f01d86 100644
--- a/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.vue
+++ b/jdaviz/configs/imviz/plugins/aper_phot_simple/aper_phot_simple.vue
@@ -113,6 +113,7 @@
             type="number"
             hint="Factor to convert data unit to counts, in unit of flux/counts"
             persistent-hint
+            :rules="[() => counts_factor>=0 || 'Counts conversion factor cannot be negative.']"
           >
           </v-text-field>
         </v-row>
@@ -175,7 +176,7 @@
             :results_isolated_to_plugin="true"
             @click="do_aper_phot"
             :spinner="spinner"
-            :disabled="aperture_selected === background_selected || !aperture_selected_validity.is_aperture"
+            :disabled="aperture_selected === background_selected || !aperture_selected_validity.is_aperture || counts_factor < 0"
           >
             Calculate
           </plugin-action-button>