diff --git a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
--- a/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/LegalizeVectorTypes.cpp
@@ -6241,13 +6241,34 @@
 SDValue DAGTypeLegalizer::WidenVecOp_INSERT_SUBVECTOR(SDNode *N) {
   EVT VT = N->getValueType(0);
   SDValue SubVec = N->getOperand(1);
+  EVT SubVT = SubVec.getValueType();
   SDValue InVec = N->getOperand(0);
 
   if (getTypeAction(SubVec.getValueType()) == TargetLowering::TypeWidenVector)
     SubVec = GetWidenedVector(SubVec);
 
-  if (SubVec.getValueType().knownBitsLE(VT) && InVec.isUndef() &&
-      N->getConstantOperandVal(2) == 0)
+  // Whether or not all the elements of SubVec will be inserted into valid
+  // indices of VT.
+  bool IndicesValid = false;
+  // If we statically know that VT can fit SubVT, the indices are valid.
+  if (VT.knownBitsGE(SubVT))
+    IndicesValid = true;
+  else if (VT.isScalableVector() && SubVT.isFixedLengthVector()) {
+    // Otherwise, if we're inserting a fixed vector into a scalable vector and
+    // we know the minimum vscale we can work out if it's valid ourselves.
+    Attribute Attr = DAG.getMachineFunction().getFunction().getFnAttribute(
+        Attribute::VScaleRange);
+    if (Attr.isValid()) {
+      unsigned VScaleMin = Attr.getVScaleRangeMin();
+      if (VT.getSizeInBits().getKnownMinValue() * VScaleMin >=
+          SubVT.getFixedSizeInBits())
+        IndicesValid = true;
+    }
+  }
+
+  // We need to make sure that the indices are still valid, otherwise we might
+  // widen what was previously well-defined to something undefined.
+  if (IndicesValid && InVec.isUndef() && N->getConstantOperandVal(2) == 0)
     return DAG.getNode(ISD::INSERT_SUBVECTOR, SDLoc(N), VT, InVec, SubVec,
                        N->getOperand(2));
 
diff --git a/llvm/test/CodeGen/RISCV/rvv/insert-subvector.ll b/llvm/test/CodeGen/RISCV/rvv/insert-subvector.ll
--- a/llvm/test/CodeGen/RISCV/rvv/insert-subvector.ll
+++ b/llvm/test/CodeGen/RISCV/rvv/insert-subvector.ll
@@ -495,6 +495,18 @@
   ret void
 }
 
+; We should be able to widen the <3 x i64> subvector to a <4 x i64> here because
+; we know that the minimum vscale is 2
+define <vscale x 2 x i64> @insert_nxv2i64_nxv3i64(<3 x i64> %sv) #0 {
+; CHECK-LABEL: insert_nxv2i64_nxv3i64:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ret
+  %vec = call <vscale x 2 x i64> @llvm.vector.insert.nxv2i64.v3i64(<vscale x 2 x i64> undef, <3 x i64> %sv, i64 0)
+  ret <vscale x 2 x i64> %vec
+}
+
+attributes #0 = { vscale_range(2,1024) }
+
 declare <vscale x 4 x i1> @llvm.vector.insert.nxv1i1.nxv4i1(<vscale x 4 x i1>, <vscale x 1 x i1>, i64)
 declare <vscale x 32 x i1> @llvm.vector.insert.nxv8i1.nxv32i1(<vscale x 32 x i1>, <vscale x 8 x i1>, i64)
 
@@ -512,3 +524,5 @@
 declare <vscale x 16 x i32> @llvm.vector.insert.nxv2i32.nxv16i32(<vscale x 16 x i32>, <vscale x 2 x i32>, i64 %idx)
 declare <vscale x 16 x i32> @llvm.vector.insert.nxv4i32.nxv16i32(<vscale x 16 x i32>, <vscale x 4 x i32>, i64 %idx)
 declare <vscale x 16 x i32> @llvm.vector.insert.nxv8i32.nxv16i32(<vscale x 16 x i32>, <vscale x 8 x i32>, i64 %idx)
+
+declare <vscale x 2 x i64> @llvm.vector.insert.nxv2i64.v3i64(<vscale x 2 x i64>, <3 x i64>, i64 %idx)