diff --git a/Tests/images/bc5_snorm.dds b/Tests/images/bc5_snorm.dds
new file mode 100644
index 000000000..0b999eed3
Binary files /dev/null and b/Tests/images/bc5_snorm.dds differ
diff --git a/Tests/images/bc5_snorm.png b/Tests/images/bc5_snorm.png
new file mode 100644
index 000000000..39d7811bf
Binary files /dev/null and b/Tests/images/bc5_snorm.png differ
diff --git a/Tests/test_file_dds.py b/Tests/test_file_dds.py
index ba9220ca7..c9414fb9a 100644
--- a/Tests/test_file_dds.py
+++ b/Tests/test_file_dds.py
@@ -10,7 +10,8 @@ from .helper import assert_image_equal, assert_image_equal_tofile
 TEST_FILE_DXT1 = "Tests/images/dxt1-rgb-4bbp-noalpha_MipMaps-1.dds"
 TEST_FILE_DXT3 = "Tests/images/dxt3-argb-8bbp-explicitalpha_MipMaps-1.dds"
 TEST_FILE_DXT5 = "Tests/images/dxt5-argb-8bbp-interpolatedalpha_MipMaps-1.dds"
-TEST_FILE_DX10_BC5 = "Tests/images/bc5_unorm.dds"
+TEST_FILE_DX10_BC5_UNORM = "Tests/images/bc5_unorm.dds"
+TEST_FILE_DX10_BC5_SNORM = "Tests/images/bc5_snorm.dds"
 TEST_FILE_DX10_BC7 = "Tests/images/bc7-argb-8bpp_MipMaps-1.dds"
 TEST_FILE_DX10_BC7_UNORM_SRGB = "Tests/images/DXGI_FORMAT_BC7_UNORM_SRGB.dds"
 TEST_FILE_DX10_R8G8B8A8 = "Tests/images/argb-32bpp_MipMaps-1.dds"
@@ -59,17 +60,30 @@ def test_sanity_dxt3():
         assert_image_equal_tofile(im, TEST_FILE_DXT3.replace(".dds", ".png"))
 
 
-def test_dx10_bc5():
+def test_dx10_bc5_unorm():
     """Check DX10 images can be opened"""
 
-    with Image.open(TEST_FILE_DX10_BC5) as im:
+    with Image.open(TEST_FILE_DX10_BC5_UNORM) as im:
         im.load()
 
         assert im.format == "DDS"
         assert im.mode == "RGB"
         assert im.size == (256, 256)
 
-        assert_image_equal_tofile(im, TEST_FILE_DX10_BC5.replace(".dds", ".png"))
+        assert_image_equal_tofile(im, TEST_FILE_DX10_BC5_UNORM.replace(".dds", ".png"))
+
+
+def test_dx10_bc5_snorm():
+    """Check DX10 images can be opened"""
+
+    with Image.open(TEST_FILE_DX10_BC5_SNORM) as im:
+        im.load()
+
+        assert im.format == "DDS"
+        assert im.mode == "RGB"
+        assert im.size == (256, 256)
+
+        assert_image_equal_tofile(im, TEST_FILE_DX10_BC5_SNORM.replace(".dds", ".png"))
 
 
 def test_dx10_bc7():
diff --git a/src/PIL/DdsImagePlugin.py b/src/PIL/DdsImagePlugin.py
index 6be43917a..dba4a5d72 100644
--- a/src/PIL/DdsImagePlugin.py
+++ b/src/PIL/DdsImagePlugin.py
@@ -151,6 +151,10 @@ class DdsImageFile(ImageFile.ImageFile):
             elif fourcc == b"DXT5":
                 self.pixel_format = "DXT5"
                 n = 3
+            elif fourcc == b"BC5S":
+                self.pixel_format = "BC5S"
+                n = 5
+                self.mode = "RGB"
             elif fourcc == b"DX10":
                 data_start += 20
                 # ignoring flags which pertain to volume textures and cubemaps
@@ -183,7 +187,9 @@ class DdsImageFile(ImageFile.ImageFile):
             else:
                 raise NotImplementedError(f"Unimplemented pixel format {repr(fourcc)}")
 
-            self.tile = [("bcn", (0, 0) + self.size, data_start, (n))]
+            self.tile = [
+                ("bcn", (0, 0) + self.size, data_start, (n, self.pixel_format))
+            ]
 
     def load_seek(self, pos):
         pass
diff --git a/src/decode.c b/src/decode.c
index c416f8a2c..2557bb36f 100644
--- a/src/decode.c
+++ b/src/decode.c
@@ -34,9 +34,10 @@
 
 #include "libImaging/Imaging.h"
 
+#include "libImaging/Bit.h"
+#include "libImaging/Bcn.h"
 #include "libImaging/Gif.h"
 #include "libImaging/Raw.h"
-#include "libImaging/Bit.h"
 #include "libImaging/Sgi.h"
 
 /* -------------------------------------------------------------------- */
@@ -359,8 +360,8 @@ PyImaging_BcnDecoderNew(PyObject *self, PyObject *args) {
     char *mode;
     char *actual;
     int n = 0;
-    int ystep = 1;
-    if (!PyArg_ParseTuple(args, "s|ii", &mode, &n, &ystep)) {
+    char *pixel_format = "";
+    if (!PyArg_ParseTuple(args, "si|s", &mode, &n, &pixel_format)) {
         return NULL;
     }
 
@@ -391,14 +392,14 @@ PyImaging_BcnDecoderNew(PyObject *self, PyObject *args) {
         return NULL;
     }
 
-    decoder = PyImaging_DecoderNew(0);
+    decoder = PyImaging_DecoderNew(sizeof(char *));
     if (decoder == NULL) {
         return NULL;
     }
 
     decoder->decode = ImagingBcnDecode;
     decoder->state.state = n;
-    decoder->state.ystep = ystep;
+    ((BCNSTATE *)decoder->state.context)->pixel_format = pixel_format;
 
     return (PyObject *)decoder;
 }
diff --git a/src/libImaging/Bcn.h b/src/libImaging/Bcn.h
new file mode 100644
index 000000000..1a6fbee45
--- /dev/null
+++ b/src/libImaging/Bcn.h
@@ -0,0 +1,3 @@
+typedef struct {
+    char *pixel_format;
+} BCNSTATE;
diff --git a/src/libImaging/BcnDecode.c b/src/libImaging/BcnDecode.c
index 5e8b456b8..22b36eb7a 100644
--- a/src/libImaging/BcnDecode.c
+++ b/src/libImaging/BcnDecode.c
@@ -13,6 +13,8 @@
 
 #include "Imaging.h"
 
+#include "Bcn.h"
+
 typedef struct {
     UINT8 r, g, b, a;
 } rgba;
@@ -35,6 +37,11 @@ typedef struct {
     UINT8 lut[6];
 } bc3_alpha;
 
+typedef struct {
+    INT8 a0, a1;
+    UINT8 lut[6];
+} bc5s_alpha;
+
 #define LOAD16(p) (p)[0] | ((p)[1] << 8)
 
 #define LOAD32(p) (p)[0] | ((p)[1] << 8) | ((p)[2] << 16) | ((p)[3] << 24)
@@ -46,11 +53,6 @@ bc1_color_load(bc1_color *dst, const UINT8 *src) {
     dst->lut = LOAD32(src + 4);
 }
 
-static void
-bc3_alpha_load(bc3_alpha *dst, const UINT8 *src) {
-    memcpy(dst, src, sizeof(bc3_alpha));
-}
-
 static rgba
 decode_565(UINT16 x) {
     rgba c;
@@ -113,15 +115,26 @@ decode_bc1_color(rgba *dst, const UINT8 *src, int separate_alpha) {
 }
 
 static void
-decode_bc3_alpha(char *dst, const UINT8 *src, int stride, int o) {
-    bc3_alpha b;
+decode_bc3_alpha(char *dst, const UINT8 *src, int stride, int o, int sign) {
     UINT16 a0, a1;
     UINT8 a[8];
-    int n, lut, aw;
-    bc3_alpha_load(&b, src);
+    int n, lut1, lut2, aw;
+    if (sign == 1) {
+        bc5s_alpha b;
+        memcpy(&b, src, sizeof(bc5s_alpha));
+        a0 = (b.a0 + 255) / 2;
+        a1 = (b.a1 + 255) / 2;
+        lut1 = b.lut[0] | (b.lut[1] << 8) | (b.lut[2] << 16);
+        lut2 = b.lut[3] | (b.lut[4] << 8) | (b.lut[5] << 16);
+    } else {
+        bc3_alpha b;
+        memcpy(&b, src, sizeof(bc3_alpha));
+        a0 = b.a0;
+        a1 = b.a1;
+        lut1 = b.lut[0] | (b.lut[1] << 8) | (b.lut[2] << 16);
+        lut2 = b.lut[3] | (b.lut[4] << 8) | (b.lut[5] << 16);
+    }
 
-    a0 = b.a0;
-    a1 = b.a1;
     a[0] = (UINT8)a0;
     a[1] = (UINT8)a1;
     if (a0 > a1) {
@@ -139,14 +152,12 @@ decode_bc3_alpha(char *dst, const UINT8 *src, int stride, int o) {
         a[6] = 0;
         a[7] = 0xff;
     }
-    lut = b.lut[0] | (b.lut[1] << 8) | (b.lut[2] << 16);
     for (n = 0; n < 8; n++) {
-        aw = 7 & (lut >> (3 * n));
+        aw = 7 & (lut1 >> (3 * n));
         dst[stride * n + o] = a[aw];
     }
-    lut = b.lut[3] | (b.lut[4] << 8) | (b.lut[5] << 16);
     for (n = 0; n < 8; n++) {
-        aw = 7 & (lut >> (3 * n));
+        aw = 7 & (lut2 >> (3 * n));
         dst[stride * (8 + n) + o] = a[aw];
     }
 }
@@ -172,18 +183,18 @@ decode_bc2_block(rgba *col, const UINT8 *src) {
 static void
 decode_bc3_block(rgba *col, const UINT8 *src) {
     decode_bc1_color(col, src + 8, 1);
-    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 3);
+    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 3, 0);
 }
 
 static void
 decode_bc4_block(lum *col, const UINT8 *src) {
-    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 0);
+    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 0, 0);
 }
 
 static void
-decode_bc5_block(rgba *col, const UINT8 *src) {
-    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 0);
-    decode_bc3_alpha((char *)col, src + 8, sizeof(col[0]), 1);
+decode_bc5_block(rgba *col, const UINT8 *src, int sign) {
+    decode_bc3_alpha((char *)col, src, sizeof(col[0]), 0, sign);
+    decode_bc3_alpha((char *)col, src + 8, sizeof(col[0]), 1, sign);
 }
 
 /* BC6 and BC7 are described here:
@@ -813,7 +824,7 @@ put_block(Imaging im, ImagingCodecState state, const char *col, int sz, int C) {
 
 static int
 decode_bcn(
-    Imaging im, ImagingCodecState state, const UINT8 *src, int bytes, int N, int C) {
+    Imaging im, ImagingCodecState state, const UINT8 *src, int bytes, int N, int C, char *pixel_format) {
     int ymax = state->ysize + state->yoff;
     const UINT8 *ptr = src;
     switch (N) {
@@ -836,7 +847,19 @@ decode_bcn(
         DECODE_LOOP(2, 16, rgba);
         DECODE_LOOP(3, 16, rgba);
         DECODE_LOOP(4, 8, lum);
-        DECODE_LOOP(5, 16, rgba);
+        case 5:
+            while (bytes >= 16) {
+                rgba col[16];
+                memset(col, 0, 16 * sizeof(col[0]));
+                decode_bc5_block(col, ptr, strcmp(pixel_format, "BC5S") == 0 ? 1 : 0);
+                put_block(im, state, (const char *)col, sizeof(col[0]), C);
+                ptr += 16;
+                bytes -= 16;
+                if (state->y >= ymax) {
+                    return -1;
+                }
+            }
+            break;
         case 6:
             while (bytes >= 16) {
                 rgb32f col[16];
@@ -849,7 +872,7 @@ decode_bcn(
                 }
             }
             break;
-            DECODE_LOOP(7, 16, rgba);
+        DECODE_LOOP(7, 16, rgba);
 #undef DECODE_LOOP
     }
     return (int)(ptr - src);
@@ -860,9 +883,7 @@ ImagingBcnDecode(Imaging im, ImagingCodecState state, UINT8 *buf, Py_ssize_t byt
     int N = state->state & 0xf;
     int width = state->xsize;
     int height = state->ysize;
-    if ((width & 3) | (height & 3)) {
-        return decode_bcn(im, state, buf, bytes, N, 1);
-    } else {
-        return decode_bcn(im, state, buf, bytes, N, 0);
-    }
+    int C = (width & 3) | (height & 3) ? 1 : 0;
+    char *pixel_format = ((BCNSTATE *)state->context)->pixel_format;
+    return decode_bcn(im, state, buf, bytes, N, C, pixel_format);
 }