diff --git a/components/script/dom/htmlselectelement.rs b/components/script/dom/htmlselectelement.rs
index 03d10e117f4e..c44dac71000f 100644
--- a/components/script/dom/htmlselectelement.rs
+++ b/components/script/dom/htmlselectelement.rs
@@ -299,6 +299,36 @@ impl HTMLSelectElementMethods for HTMLSelectElement {
             opt.set_selectedness(false);
         }
     }
+
+    // https://html.spec.whatwg.org/multipage/#dom-select-selectedindex
+    fn SelectedIndex(&self) -> i32 {
+        self.upcast::<Node>()
+            .traverse_preorder()
+            .filter_map(Root::downcast::<HTMLOptionElement>)
+            .enumerate()
+            .filter(|&(_, ref opt_elem)| opt_elem.Selected())
+            .map(|(i, _)| i as i32)
+            .next()
+            .unwrap_or(-1)
+    }
+
+    // https://html.spec.whatwg.org/multipage/#dom-select-selectedindex
+    fn SetSelectedIndex(&self, index: i32) {
+        let mut opt_iter = self.upcast::<Node>()
+                               .traverse_preorder()
+                               .filter_map(Root::downcast::<HTMLOptionElement>);
+        for opt in opt_iter.by_ref().take(index as usize) {
+            opt.set_selectedness(false);
+        }
+        if let Some(opt) = opt_iter.next() {
+            opt.set_selectedness(true);
+            opt.set_dirtiness(true);
+            // Reset remaining <option> elements
+            for opt in opt_iter {
+                opt.set_selectedness(false);
+            }
+        }
+    }
 }
 
 impl VirtualMethods for HTMLSelectElement {
diff --git a/components/script/dom/webidls/HTMLSelectElement.webidl b/components/script/dom/webidls/HTMLSelectElement.webidl
index a8fd2803c44b..543c3267c64e 100644
--- a/components/script/dom/webidls/HTMLSelectElement.webidl
+++ b/components/script/dom/webidls/HTMLSelectElement.webidl
@@ -25,7 +25,7 @@ interface HTMLSelectElement : HTMLElement {
   //setter void (unsigned long index, HTMLOptionElement? option);
 
   //readonly attribute HTMLCollection selectedOptions;
-  //         attribute long selectedIndex;
+  attribute long selectedIndex;
   attribute DOMString value;
 
   //readonly attribute boolean willValidate;
diff --git a/tests/wpt/metadata/MANIFEST.json b/tests/wpt/metadata/MANIFEST.json
index a57203173862..bfc8b84612d0 100644
--- a/tests/wpt/metadata/MANIFEST.json
+++ b/tests/wpt/metadata/MANIFEST.json
@@ -37763,6 +37763,12 @@
             "url": "/html/semantics/forms/the-select-element/common-HTMLOptionsCollection-add.html"
           }
         ],
+        "html/semantics/forms/the-select-element/selected-index.html": [
+          {
+            "path": "html/semantics/forms/the-select-element/selected-index.html",
+            "url": "/html/semantics/forms/the-select-element/selected-index.html"
+          }
+        ],
         "html/webappapis/scripting/events/uncompiled_event_handler_with_scripting_disabled.html": [
           {
             "path": "html/webappapis/scripting/events/uncompiled_event_handler_with_scripting_disabled.html",
diff --git a/tests/wpt/metadata/html/dom/interfaces.html.ini b/tests/wpt/metadata/html/dom/interfaces.html.ini
index 98ace57ae686..67181fad83da 100644
--- a/tests/wpt/metadata/html/dom/interfaces.html.ini
+++ b/tests/wpt/metadata/html/dom/interfaces.html.ini
@@ -3708,9 +3708,6 @@
   [HTMLSelectElement interface: attribute selectedOptions]
     expected: FAIL
 
-  [HTMLSelectElement interface: attribute selectedIndex]
-    expected: FAIL
-
   [HTMLSelectElement interface: attribute willValidate]
     expected: FAIL
 
@@ -3738,9 +3735,6 @@
   [HTMLSelectElement interface: document.createElement("select") must inherit property "selectedOptions" with the proper type (17)]
     expected: FAIL
 
-  [HTMLSelectElement interface: document.createElement("select") must inherit property "selectedIndex" with the proper type (18)]
-    expected: FAIL
-
   [HTMLSelectElement interface: document.createElement("select") must inherit property "willValidate" with the proper type (20)]
     expected: FAIL
 
diff --git a/tests/wpt/web-platform-tests/html/semantics/forms/the-select-element/selected-index.html b/tests/wpt/web-platform-tests/html/semantics/forms/the-select-element/selected-index.html
new file mode 100644
index 000000000000..1be5378cdf4f
--- /dev/null
+++ b/tests/wpt/web-platform-tests/html/semantics/forms/the-select-element/selected-index.html
@@ -0,0 +1,73 @@
+<!doctype html>
+<meta charset=utf-8>
+<title>HTMLSelectElement selectedIndex</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<div id=log></div>
+
+<form id=form>
+  <select id=empty></select>
+
+  <select id=default>
+    <option></option>
+    <option></option>
+    <option></option>
+    <option></option>
+    <option></option>
+  </select>
+
+  <select id=disabled>
+    <option disabled></option>
+    <option></option>
+  </select>
+
+  <select id=selected>
+    <option></option>
+    <option selected></option>
+  </select>
+</form>
+
+<script>
+test(function () {
+  var select = document.getElementById('empty');
+  assert_equals(select.selectedIndex, -1);
+}, "get empty");
+
+test(function () {
+  var select = document.getElementById('default');
+  assert_equals(select.selectedIndex, 0);
+}, "get default");
+
+test(function () {
+  var select = document.getElementById('disabled');
+  assert_equals(select.selectedIndex, 1);
+}, "get disabled");
+
+test(function () {
+  var select = document.getElementById('selected');
+  assert_equals(select.selectedIndex, 1);
+}, "get unselected");
+
+test(function () {
+  var select = document.getElementById('empty');
+  select.selectedIndex = 1;
+  assert_equals(select.selectedIndex, -1);
+}, "set empty");
+
+test(function () {
+  var select = document.getElementById('default');
+  assert_equals(select.selectedIndex, 0);
+  select.selectedIndex = 2;
+  assert_equals(select.selectedIndex, 2);
+}, "set");
+
+test(function () {
+  var select = document.getElementById('selected');
+  var form = document.getElementById('form');
+  assert_equals(select.selectedIndex, 1);
+  select.selectedIndex = 0;
+  assert_equals(select.selectedIndex, 0);
+  form.reset();
+  assert_equals(select.selectedIndex, 1);
+}, "set and reset");
+</script>