diff --git a/Tests/test_box_blur.py b/Tests/test_box_blur.py
index 391e00a23..841aefcef 100644
--- a/Tests/test_box_blur.py
+++ b/Tests/test_box_blur.py
@@ -119,3 +119,42 @@ class TestBoxBlur(PillowTestCase):
             ],
             delta=1,
         )
+
+    def test_radius_bigger_then_half(self):
+        self.assertImage(
+            box_blur(sample, 3),
+            [
+                [144, 145, 142, 128, 114, 115, 117],
+                [148, 145, 137, 122, 109, 111, 112],
+                [152, 145, 131, 117, 103, 107, 108],
+                [156, 144, 126, 111, 97,  102, 103],
+                [160, 144, 121, 106, 92,  98,  99 ],
+            ],
+            delta=1,
+        )
+
+    def test_radius_bigger_then_width(self):
+        self.assertImage(
+            box_blur(sample, 10),
+            [
+                [158, 153, 147, 141, 135, 129, 123],
+                [159, 153, 147, 141, 136, 130, 124],
+                [159, 154, 148, 142, 136, 130, 124],
+                [160, 154, 148, 142, 137, 131, 125],
+                [160, 155, 149, 143, 137, 131, 125],
+            ],
+            delta=0,
+        )
+
+    def test_exteme_large_radius(self):
+        self.assertImage(
+            box_blur(sample, 600),
+            [
+                [162, 162, 162, 162, 162, 162, 162],
+                [162, 162, 162, 162, 162, 162, 162],
+                [162, 162, 162, 162, 162, 162, 162],
+                [162, 162, 162, 162, 162, 162, 162],
+                [162, 162, 162, 162, 162, 162, 162],
+            ],
+            delta=1,
+        )
diff --git a/libImaging/BoxBlur.c b/libImaging/BoxBlur.c
index 7a3909adb..29b89dc9e 100644
--- a/libImaging/BoxBlur.c
+++ b/libImaging/BoxBlur.c
@@ -57,12 +57,18 @@ HorizontalBoxBlur32(Imaging im, Imaging imOut, float floatRadius)
         acc[1] = line[0][1] * (radius + 1);
         acc[2] = line[0][2] * (radius + 1);
         acc[3] = line[0][3] * (radius + 1);
-        for (pix = 0; pix < radius; pix++) {
+        /* As radius can be bigger than xsize, iterate to edgeA -1. */
+        for (pix = 0; pix < edgeA - 1; pix++) {
             acc[0] += line[pix][0];
             acc[1] += line[pix][1];
             acc[2] += line[pix][2];
             acc[3] += line[pix][3];
         }
+        /* Then multiply remainder to last x. */
+        acc[0] += line[lastx][0] * (radius - edgeA + 1);
+        acc[1] += line[lastx][1] * (radius - edgeA + 1);
+        acc[2] += line[lastx][2] * (radius - edgeA + 1);
+        acc[3] += line[lastx][3] * (radius - edgeA + 1);
 
         if (edgeA <= edgeB)
         {
@@ -140,9 +146,10 @@ HorizontalBoxBlur8(Imaging im, Imaging imOut, float floatRadius)
         line = im->image8[y];
 
         acc = line[0] * (radius + 1);
-        for (pix = 0; pix < radius; pix++) {
+        for (pix = 0; pix < edgeA - 1; pix++) {
             acc += line[pix];
         }
+        acc += line[lastx] * (radius - edgeA + 1);
 
         if (edgeA <= edgeB)
         {