diff --git a/sorting/counting_sort.rb b/sorting/counting_sort.rb
new file mode 100644
index 0000000..a50f636
--- /dev/null
+++ b/sorting/counting_sort.rb
@@ -0,0 +1,26 @@
+##
+# Given a non-negative integer value_upper_bound and an array of integers arr with values between 0 and value_upper_bound,
+# returns a sorted copy of the input array.
+# When value_upper_bound = O(arr.length), sorting runs in O(arr.length).
+
+def counting_sort(arr, value_upper_bound)
+    if !value_upper_bound.integer? || value_upper_bound < 0
+        raise ArgumentError.new("counting_sort must be invoked with integer value_upper_bound >= 0")
+    end
+    if !arr.all? { |elem| elem.integer? && elem.between?(0, value_upper_bound) }
+        raise ArgumentError.new("counting_sort must be invoked with integer array elements in (0..value_upper_bound)")
+    end
+    sorted_arr = Array.new(arr.length) { 0 }
+    tmp_arr = Array.new(value_upper_bound+1) { 0 }
+    for elem in arr
+        tmp_arr[elem] += 1
+    end
+    for i in 1..value_upper_bound
+        tmp_arr[i] += tmp_arr[i-1]
+    end
+    arr.reverse_each do |elem|
+        sorted_arr[tmp_arr[elem]-1] = elem
+        tmp_arr[elem] -= 1
+    end
+    sorted_arr
+end
diff --git a/sorting/counting_sort_test.rb b/sorting/counting_sort_test.rb
new file mode 100644
index 0000000..8cabfba
--- /dev/null
+++ b/sorting/counting_sort_test.rb
@@ -0,0 +1,42 @@
+require 'minitest/autorun'
+require_relative 'counting_sort'
+
+class TestCountingSort < Minitest::Test
+  def test_empty_array_given_empty_array
+    assert counting_sort([], 1).empty?
+  end
+  
+  def test_array_sorted_correctly
+    assert counting_sort([1, 5, 3, 0, 4, 2, 4], 5) == [0, 1, 2, 3, 4, 4, 5]
+  end
+  
+  def test_exception_given_non_integer_upper_bound
+    assert_raises ArgumentError do
+        counting_sort([1, 3, 2], 5.5)
+    end
+  end
+  
+  def test_exception_given_negative_upper_bound
+    assert_raises ArgumentError do
+        counting_sort([1, 3, 2], -1)
+    end
+  end
+  
+  def test_exception_given_non_integer_elements
+    assert_raises ArgumentError do
+        counting_sort([1, 3, 2.5], 5)
+    end
+  end
+  
+  def test_exception_given_negative_elements
+    assert_raises ArgumentError do
+        counting_sort([1, 3, -2], 5)
+    end
+  end
+  
+  def test_exception_given_elements_above_upper_bound
+    assert_raises ArgumentError do
+        counting_sort([1, 3, 6], 5)
+    end
+  end
+end