diff --git a/javalib/source/src/java/util/Formattable.scala b/javalib/source/src/java/util/Formattable.scala
new file mode 100644
index 0000000000..e651fbba24
--- /dev/null
+++ b/javalib/source/src/java/util/Formattable.scala
@@ -0,0 +1,5 @@
+package java.util
+
+trait Formattable {
+  def formatTo(formatter: Formatter, flags: Int, width: Int, precision: Int): Unit
+}
diff --git a/javalib/source/src/java/util/FormattableFlags.scala b/javalib/source/src/java/util/FormattableFlags.scala
new file mode 100644
index 0000000000..de3e8e3ab9
--- /dev/null
+++ b/javalib/source/src/java/util/FormattableFlags.scala
@@ -0,0 +1,7 @@
+package java.util
+
+object FormattableFlags {
+  val ALTERNATE = 4
+  val LEFT_JUSTIFY = 1
+  val UPPERCASE = 2
+}
diff --git a/javalib/source/src/java/util/Formatter.scala b/javalib/source/src/java/util/Formatter.scala
index a9c68983cd..b0d420c5fb 100644
--- a/javalib/source/src/java/util/Formatter.scala
+++ b/javalib/source/src/java/util/Formatter.scala
@@ -72,8 +72,9 @@ final class Formatter(private val dest: Appendable) extends Closeable with Flush
           def hasFlag(flag: js.String) = flags.indexOf(flag) >= 0
 
           val widthStr = matchResult(3)
+          val hasWidth = !(!widthStr)
           val width =
-            if (!(!widthStr)) js.parseInt(widthStr)
+            if (hasWidth) js.parseInt(widthStr)
             else (0: js.Number)
 
           val precisionStr = matchResult(4)
@@ -175,10 +176,22 @@ final class Formatter(private val dest: Appendable) extends Closeable with Flush
               if (arg eq null) "null"
               else Integer.toHexString(arg.hashCode)
             }
-            case 's' | 'S' => pad {
-              val s: js.String = if (arg eq null) "null" else arg.toString()
-              if (hasPrecision) s.substring(0, precision)
-              else s
+            case 's' | 'S' => arg match {
+              case null if !hasFlag("#") => pad("null")
+              case formattable: Formattable =>
+                val flags = (
+                  (if (hasFlag("-"))       FormattableFlags.LEFT_JUSTIFY else 0) |
+                  (if (hasFlag("#"))       FormattableFlags.ALTERNATE    else 0) |
+                  (if (conversion.isUpper) FormattableFlags.UPPERCASE    else 0)
+                )
+
+                formattable.formatTo(this, flags,
+                                     if (hasWidth)     width.toInt     else -1,
+                                     if (hasPrecision) precision.toInt else -1)
+                None // no further processing
+              case t: AnyRef if !hasFlag("#") => pad(t.toString)
+              case _ =>
+                throw new FormatFlagsConversionMismatchException("#", 's')
             }
             case 'c' | 'C' =>
               pad(js.String.fromCharCode(numberArg))
diff --git a/test/src/test/scala/scala/scalajs/test/javalib/FormatterTest.scala b/test/src/test/scala/scala/scalajs/test/javalib/FormatterTest.scala
index 1710cc2934..8e17b001f8 100644
--- a/test/src/test/scala/scala/scalajs/test/javalib/FormatterTest.scala
+++ b/test/src/test/scala/scala/scalajs/test/javalib/FormatterTest.scala
@@ -11,7 +11,7 @@ package javalib
 import scala.scalajs.js
 import scala.scalajs.test.ScalaJSTest
 
-import java.util.{ Formatter, Formattable }
+import java.util.{ Formatter, Formattable, FormattableFlags }
 
 import java.lang.{
   Double  => JDouble, 
@@ -27,6 +27,28 @@ import java.lang.{
 object FormatterTest extends ScalaJSTest {
 
   class HelperClass
+  class FormattableClass extends Formattable {
+    var frm: Formatter = _
+    var flags: Int = _
+    var width: Int = _
+    var precision: Int = _
+    var calls = 0
+    def formatTo(frm: Formatter, flags: Int, width: Int, precision: Int) = {
+      this.calls += 1
+      this.flags = flags
+      this.width = width
+      this.precision = precision
+      frm.out().append("foobar")
+    }
+
+    def expectCalled(times: Int, flags: Int, width: Int, precision: Int) = {
+      expect(this.calls).toEqual(times)
+      expect(this.flags).toEqual(flags)
+      expect(this.width).toEqual(width)
+      expect(this.precision).toEqual(precision)
+    }
+
+  }
 
   def expectF(format: String, args: AnyRef*) = {
     val fmt = new Formatter()
@@ -35,6 +57,13 @@ object FormatterTest extends ScalaJSTest {
     expect(res)
   }
 
+  def expectFC(format: String, flags: Int, width: Int, precision: Int) = {
+    val fc = new FormattableClass
+    val exp = expectF(format, fc)
+    fc.expectCalled(1, flags, width, precision)
+    exp
+  }
+
   describe("java.util.Formatter") {
 
     it("should provide 'b' conversion") {
@@ -51,6 +80,19 @@ object FormatterTest extends ScalaJSTest {
       expectF("%h", null).toEqual("null")
     }
 
+    it("should provide 's' conversion") {
+      expectFC("%s", 0, -1, -1).toEqual("foobar")
+      expectFC("%-s", FormattableFlags.LEFT_JUSTIFY, -1, -1).toEqual("foobar")
+      expectFC("%-10s", FormattableFlags.LEFT_JUSTIFY, 10, -1).toEqual("foobar")
+      expectFC("%#-10.2s", FormattableFlags.LEFT_JUSTIFY |
+               FormattableFlags.ALTERNATE, 10, 2).toEqual("foobar")
+      expectFC("%#10.2S", FormattableFlags.UPPERCASE |
+               FormattableFlags.ALTERNATE, 10, 2).toEqual("foobar")
+      expectF("%10s", "hello").toEqual("     hello")
+      expectF("%-10s", "hello").toEqual("hello     ")
+      expectThrow("%#s", "hello")
+    }
+
     it("should provide 'c' conversion") {
       expectF("%-5c", new Character('!')).toEqual("!    ")
     }