diff --git a/lucene/CHANGES.txt b/lucene/CHANGES.txt
index 459b280ddbb..04f4960a376 100644
--- a/lucene/CHANGES.txt
+++ b/lucene/CHANGES.txt
@@ -283,6 +283,9 @@ Bug fixes:
 * LUCENE-8649: LatLonShape's within and disjoint queries can return false positives with
   indexed multi-shapes. (Ignacio Vera)
 
+* LUCENE-8654: Polygon2D#relateTriangle returns the wrong answer if polygon is inside
+  the triangle. (Ignacio Vera)
+
 New Features
 
 * LUCENE-8026: ExitableDirectoryReader may now time out queries that run on
diff --git a/lucene/core/src/java/org/apache/lucene/geo/EdgeTree.java b/lucene/core/src/java/org/apache/lucene/geo/EdgeTree.java
index d954f88317f..fc3540d914b 100644
--- a/lucene/core/src/java/org/apache/lucene/geo/EdgeTree.java
+++ b/lucene/core/src/java/org/apache/lucene/geo/EdgeTree.java
@@ -150,6 +150,11 @@ public abstract class EdgeTree {
     if (tree.crossesTriangle(ax, ay, bx, by, cx, cy)) {
       return Relation.CELL_CROSSES_QUERY;
     }
+
+    if (pointInTriangle(tree.lon1, tree.lat1, ax, ay, bx, by, cx, cy) == true) {
+      return Relation.CELL_CROSSES_QUERY;
+    }
+
     return Relation.CELL_OUTSIDE_QUERY;
   }
 
@@ -379,6 +384,29 @@ public abstract class EdgeTree {
     }
   }
 
+  //This should be moved when LatLonShape is moved from sandbox!
+  /**
+   * Compute whether the given x, y point is in a triangle; uses the winding order method */
+  private static boolean pointInTriangle (double x, double y, double ax, double ay, double bx, double by, double cx, double cy) {
+    double minX = StrictMath.min(ax, StrictMath.min(bx, cx));
+    double minY = StrictMath.min(ay, StrictMath.min(by, cy));
+    double maxX = StrictMath.max(ax, StrictMath.max(bx, cx));
+    double maxY = StrictMath.max(ay, StrictMath.max(by, cy));
+    //check the bounding box because if the triangle is degenerated, e.g points and lines, we need to filter out
+    //coplanar points that are not part of the triangle.
+    if (x >= minX && x <= maxX && y >= minY && y <= maxY ) {
+      int a = orient(x, y, ax, ay, bx, by);
+      int b = orient(x, y, bx, by, cx, cy);
+      if (a == 0 || b == 0 || a < 0 == b < 0) {
+        int c = orient(x, y, cx, cy, ax, ay);
+        return c == 0 || (c < 0 == (b < 0 || a < 0));
+      }
+      return false;
+    } else {
+      return false;
+    }
+  }
+
   /**
    * Creates an edge interval tree from a set of geometry vertices.
    * @return root node of the tree.
diff --git a/lucene/core/src/test/org/apache/lucene/geo/TestPolygon2D.java b/lucene/core/src/test/org/apache/lucene/geo/TestPolygon2D.java
index 053f008b902..f85536abd1c 100644
--- a/lucene/core/src/test/org/apache/lucene/geo/TestPolygon2D.java
+++ b/lucene/core/src/test/org/apache/lucene/geo/TestPolygon2D.java
@@ -312,6 +312,12 @@ public class TestPolygon2D extends LuceneTestCase {
     }
   }
 
+  public void testRelateTriangleContainsPolygon() {
+    Polygon polygon = new Polygon(new double[]{0, 0, 1, 1, 0}, new double[]{0, 1, 1, 0, 0});
+    Polygon2D impl = Polygon2D.create(polygon);
+    assertEquals(Relation.CELL_CROSSES_QUERY, impl.relateTriangle(-10 , -1, 2, -1, 10, 10));
+  }
+
   // test
   public void testRelateTriangleEdgeCases() {
     for (int i = 0; i < 100; ++i) {