diff --git a/src/mscorlib/model.xml b/src/mscorlib/model.xml
index 71c8cab22ec3..7abb94cd4cd5 100644
--- a/src/mscorlib/model.xml
+++ b/src/mscorlib/model.xml
@@ -7855,6 +7855,10 @@
       <Member Name="AppendFormat(System.String,System.Object,System.Object)" />
       <Member Name="AppendFormat(System.String,System.Object,System.Object,System.Object)" />
       <Member Name="AppendFormat(System.String,System.Object[])" />
+      <Member Name="AppendJoin&lt;T&gt;(System.Char,System.Collections.Generic.IEnumerable&lt;T&gt;)" />
+      <Member Name="AppendJoin&lt;T&gt;(System.Char,T[])" />
+      <Member Name="AppendJoin&lt;T&gt;(System.String,System.Collections.Generic.IEnumerable&lt;T&gt;)" />
+      <Member Name="AppendJoin&lt;T&gt;(System.String,T[])" />
       <Member Name="AppendLine" />
       <Member Name="AppendLine(System.String)" />
       <Member Name="Clear" />
diff --git a/src/mscorlib/src/System/Text/StringBuilder.cs b/src/mscorlib/src/System/Text/StringBuilder.cs
index f525118fa4b5..e81d90c72281 100644
--- a/src/mscorlib/src/System/Text/StringBuilder.cs
+++ b/src/mscorlib/src/System/Text/StringBuilder.cs
@@ -21,6 +21,7 @@ namespace System.Text {
     using System.Threading;
     using System.Globalization;
     using System.Diagnostics.Contracts;
+    using System.Collections.Generic;
 
     // This class represents a mutable string.  It is convenient for situations in
     // which it is desirable to modify a string, perhaps by removing, replacing, or 
@@ -1011,6 +1012,88 @@ public StringBuilder Append(char[] value) {
             return this;
         }
 
+        // Append joined values with a separator between each value.
+        public unsafe StringBuilder AppendJoin<T>(char separator, params T[] values)
+        {
+            // Defer argument validation to the internal function
+            return AppendJoinCore(&separator, 1, values);
+        }
+
+        public unsafe StringBuilder AppendJoin<T>(string separator, params T[] values)
+        {
+            separator = separator ?? string.Empty;
+            fixed (char* pSeparator = separator)
+            {
+                // Defer argument validation to the internal function
+                return AppendJoinCore(pSeparator, separator.Length, values);
+            }
+        }
+
+        public unsafe StringBuilder AppendJoin<T>(char separator, IEnumerable<T> values)
+        {
+            // Defer argument validation to the internal function
+            return AppendJoinCore(&separator, 1, values);
+        }
+
+        public unsafe StringBuilder AppendJoin<T>(string separator, IEnumerable<T> values)
+        {
+            separator = separator ?? string.Empty;
+            fixed (char* pSeparator = separator)
+            {
+                // Defer argument validation to the internal function
+                return AppendJoinCore(pSeparator, separator.Length, values);
+            }
+        }
+
+        private unsafe StringBuilder AppendJoinCore<T>(char* separator, int separatorLength, params T[] values)
+        {
+            if (values == null)
+                throw new ArgumentNullException(nameof(values));
+            Contract.Ensures(Contract.Result<StringBuilder>() != null);
+
+            if (values.Length == 0)
+                return this;
+
+            var value = values[0];
+            if (value != null)
+                Append(value.ToString());
+
+            for (var i = 1; i < values.Length; i++)
+            {
+                Append(separator, separatorLength);
+                value = values[i];
+                if (value != null)
+                    Append(value.ToString());
+            }
+            return this;
+        }
+
+        private unsafe StringBuilder AppendJoinCore<T>(char* separator, int separatorLength, IEnumerable<T> values)
+        {
+            if (values == null)
+                throw new ArgumentNullException(nameof(values));
+            Contract.Ensures(Contract.Result<StringBuilder>() != null);
+
+            using (var en = values.GetEnumerator())
+            {
+                if (!en.MoveNext())
+                    return this;
+
+                var value = en.Current;
+                if (value != null)
+                    Append(value.ToString());
+
+                while (en.MoveNext())
+                {
+                    Append(separator, separatorLength);
+                    value = en.Current;
+                    if (value != null)
+                        Append(value.ToString());
+                }
+            }
+            return this;
+        }
+
         /*====================================Insert====================================
         **
         ==============================================================================*/