diff --git a/lib/securerandom.rb b/lib/securerandom.rb
index 476f5e1..f04c6e2 100644
--- a/lib/securerandom.rb
+++ b/lib/securerandom.rb
@@ -297,4 +297,23 @@ module SecureRandom
     ary[3] = (ary[3] & 0x3fff) | 0x8000
     "%08x-%04x-%04x-%04x-%04x%08x" % ary
   end
+
+  # SecureRandom.alphanumeric generates a random alphanumeric string.
+  #
+  # The argument _n_ specifies the length, in bytes, of the random number
+  # to be generated. The length of the result string is about 4/3 of _n_.
+  #
+  # If _n_ is not specified or is nil, 16 is assumed.
+  # It may be larger in the future.
+  #
+  # The result may contain A-Z, a-z and 0-9.
+  #
+  #   p SecureRandom.alphanumeric #=> "2BuBuLf3WfSKyQbRccA"
+  #   p SecureRandom.alphanumeric #=> "6BbW0pxO0YENxn38HMUbcQ"
+  #
+  # If a secure random number generator is not available,
+  # +NotImplementedError+ is raised.
+  def self.alphanumeric(n=nil)
+    [random_bytes(n)].pack("m*").delete("\n+/=")
+  end
 end
diff --git a/test/test_securerandom.rb b/test/test_securerandom.rb
index 0a78624..d49d579 100644
--- a/test/test_securerandom.rb
+++ b/test/test_securerandom.rb
@@ -164,6 +164,13 @@ end
     end
   end
 
+  def test_alphanumeric
+    65.times do |idx|
+      an = @it.alphanumeric
+      assert_match(/^[0-9a-zA-Z]+$/, an)
+    end
+  end
+
   def protect
     begin
       yield