diff --git a/modules/video/src/dis_flow.cpp b/modules/video/src/dis_flow.cpp
index c859555c22..5ebb3b8f45 100644
--- a/modules/video/src/dis_flow.cpp
+++ b/modules/video/src/dis_flow.cpp
@@ -818,10 +818,12 @@ void DISOpticalFlowImpl::PatchInverseSearch_ParBody::operator()(const Range &ran
     int i, j, dir;
     int start_is, end_is, start_js, end_js;
     int start_i, start_j;
-    float i_lower_limit = bsz - psz + 1.0f;
-    float i_upper_limit = bsz + dis->h - 1.0f;
-    float j_lower_limit = bsz - psz + 1.0f;
-    float j_upper_limit = bsz + dis->w - 1.0f;
+    // Clamp so the psz+1 (bilinear) read stays inside the padded I1_ext buffer even when
+    // psz > bsz; unchanged for psz <= bsz (the common case). See #20185.
+    float i_lower_limit = std::max(bsz - psz + 1.0f, 0.0f);
+    float i_upper_limit = std::min(bsz + dis->h - 1.0f, dis->h + 2.0f * bsz - 1.0f - psz);
+    float j_lower_limit = std::max(bsz - psz + 1.0f, 0.0f);
+    float j_upper_limit = std::min(bsz + dis->w - 1.0f, dis->w + 2.0f * bsz - 1.0f - psz);
     float dUx, dUy, i_I1, j_I1, w00, w01, w10, w11, dx, dy;
 
 #define INIT_BILINEAR_WEIGHTS(Ux, Uy) \
diff --git a/modules/video/test/test_OF_accuracy.cpp b/modules/video/test/test_OF_accuracy.cpp
index cf51cd1dc4..063c221e1a 100644
--- a/modules/video/test/test_OF_accuracy.cpp
+++ b/modules/video/test/test_OF_accuracy.cpp
@@ -197,4 +197,24 @@ TEST(DenseOpticalFlow_DIS, ManualCoarsestScale)
     EXPECT_EQ(dis->getCoarsestScale(), -1);
 }
 
-}} // namespace
\ No newline at end of file
+// See https://github.com/opencv/opencv/issues/20185
+TEST(DenseOpticalFlow_DIS, regression_20185_patch_larger_than_border)
+{
+    Mat prev(240, 320, CV_8UC1), next(240, 320, CV_8UC1);
+    theRNG().fill(prev, RNG::UNIFORM, 0, 256);
+    theRNG().fill(next, RNG::UNIFORM, 0, 256);
+
+    Ptr<DISOpticalFlow> dis = DISOpticalFlow::create(DISOpticalFlow::PRESET_MEDIUM);
+    dis->setPatchStride(10);
+
+    Mat flow(prev.size(), CV_32FC2, Scalar(60.f, 60.f));
+    ASSERT_NO_THROW(dis->calc(prev, next, flow));
+    EXPECT_EQ(flow.size(), prev.size());
+
+    dis->setPatchSize(25);
+    flow.setTo(Scalar(60.f, 60.f));
+    ASSERT_NO_THROW(dis->calc(prev, next, flow));
+    EXPECT_EQ(flow.size(), prev.size());
+}
+
+}} // namespace