diff --git a/bignum.c b/bignum.c
index 088e8c49b55..89b35dd1e12 100644
--- a/bignum.c
+++ b/bignum.c
@@ -7029,6 +7029,23 @@ rb_big_bit_length(VALUE big)
             INTEGER_PACK_LSWORD_FIRST|INTEGER_PACK_NATIVE_BYTE_ORDER);
 }
 
+VALUE
+rb_big_bit_count(VALUE big)
+{
+    if (BIGNUM_NEGATIVE_P(big))
+        rb_raise(rb_eArgError, "bit_count is undefined for negative integers");
+
+    BDIGIT *ds = BDIGITS(big);
+    size_t n = BIGNUM_LEN(big);
+    size_t count = 0;
+
+    while (n--) {
+        count += rb_popcount64((uint64_t)ds[n]);
+    }
+
+    return SIZET2NUM(count);
+}
+
 VALUE
 rb_big_odd_p(VALUE num)
 {
diff --git a/internal/bignum.h b/internal/bignum.h
index 7389a17c747..08f7c85b578 100644
--- a/internal/bignum.h
+++ b/internal/bignum.h
@@ -123,6 +123,7 @@ VALUE rb_big_aref2(VALUE num, VALUE beg, VALUE len);
 VALUE rb_big_abs(VALUE x);
 VALUE rb_big_size_m(VALUE big);
 VALUE rb_big_bit_length(VALUE big);
+VALUE rb_big_bit_count(VALUE big);
 VALUE rb_big_remainder(VALUE x, VALUE y);
 VALUE rb_big_gt(VALUE x, VALUE y);
 VALUE rb_big_ge(VALUE x, VALUE y);
diff --git a/internal/numeric.h b/internal/numeric.h
index a24432df1e6..ed1f6035239 100644
--- a/internal/numeric.h
+++ b/internal/numeric.h
@@ -126,6 +126,7 @@ VALUE rb_int_even_p(VALUE num);
 VALUE rb_int_odd_p(VALUE num);
 VALUE rb_int_abs(VALUE num);
 VALUE rb_int_bit_length(VALUE num);
+VALUE rb_int_bit_count(VALUE num);
 VALUE rb_int_uminus(VALUE num);
 VALUE rb_int_comp(VALUE num);
 
diff --git a/numeric.c b/numeric.c
index 90270617c18..f80a419bafb 100644
--- a/numeric.c
+++ b/numeric.c
@@ -5732,6 +5732,49 @@ rb_int_bit_length(VALUE num)
     return Qnil;
 }
 
+static VALUE
+rb_fix_bit_count(VALUE fix)
+{
+    long v = FIX2LONG(fix);
+    if (v < 0)
+        rb_raise(rb_eArgError, "bit_count is undefined for negative integers");
+    return LONG2FIX(rb_popcount_intptr((uintptr_t)v));
+}
+
+/*
+ *  call-seq:
+ *    bit_count -> integer
+ *
+ *  Returns the number of set bits (bits equal to 1) in the binary
+ *  representation of +self+, also known as the population count or
+ *  Hamming weight.
+ *
+ *    0.bit_count            # => 0
+ *    1.bit_count            # => 1
+ *    7.bit_count            # => 3
+ *    0b10101.bit_count      # => 3
+ *    255.bit_count          # => 8
+ *    (2**1000).bit_count    # => 1
+ *    (2**1000-1).bit_count  # => 1000
+ *
+ *  Raises an exception if +self+ is negative.
+ *
+ *    (-1).bit_count # Raises ArgumentError
+ *
+ */
+
+VALUE
+rb_int_bit_count(VALUE num)
+{
+    if (FIXNUM_P(num)) {
+        return rb_fix_bit_count(num);
+    }
+    else if (RB_BIGNUM_TYPE_P(num)) {
+        return rb_big_bit_count(num);
+    }
+    UNREACHABLE_RETURN(Qnil);
+}
+
 static VALUE
 rb_fix_digits(VALUE fix, long base)
 {
@@ -6610,6 +6653,7 @@ Init_Numeric(void)
     rb_define_method(rb_cInteger, ">>", rb_int_rshift, 1);
 
     rb_define_method(rb_cInteger, "digits", rb_int_digits, -1);
+    rb_define_method(rb_cInteger, "bit_count", rb_int_bit_count, 0);
 
 #define fix_to_s_static(n) do { \
         VALUE lit = rb_fstring_literal(#n); \
diff --git a/spec/ruby/core/integer/bit_count_spec.rb b/spec/ruby/core/integer/bit_count_spec.rb
new file mode 100644
index 00000000000..6fa06154aae
--- /dev/null
+++ b/spec/ruby/core/integer/bit_count_spec.rb
@@ -0,0 +1,31 @@
+require_relative '../../spec_helper'
+
+ruby_version_is "4.1" do
+  describe "Integer#bit_count" do
+    it "returns the number of set bits in the binary representation" do
+      0.bit_count.should == 0
+      1.bit_count.should == 1
+      2.bit_count.should == 1
+      3.bit_count.should == 2
+      7.bit_count.should == 3
+      0b10101.bit_count.should == 3
+      0xff.bit_count.should == 8
+      0x100.bit_count.should == 1
+      fixnum_max.bit_count.should == fixnum_max.to_s(2).count("1")
+
+      (2**1000).bit_count.should == 1
+      (2**1000 - 1).bit_count.should == 1000
+      (2**1000 + 2**500).bit_count.should == 2
+      (2**64 - 1).bit_count.should == 64
+      (2**10000 - 1).bit_count.should == 10000
+    end
+
+    it "raises an ArgumentError for a negative number" do
+      -> { -1.bit_count }.should raise_error(ArgumentError)
+      -> { -19.bit_count }.should raise_error(ArgumentError)
+      -> { fixnum_min.bit_count }.should raise_error(ArgumentError)
+      -> { (-2**1000).bit_count }.should raise_error(ArgumentError)
+      -> { (-2**1000 - 1).bit_count }.should raise_error(ArgumentError)
+    end
+  end
+end
diff --git a/test/ruby/test_integer.rb b/test/ruby/test_integer.rb
index c3d9d311c84..7753bce20c5 100644
--- a/test/ruby/test_integer.rb
+++ b/test/ruby/test_integer.rb
@@ -632,6 +632,41 @@ class TestInteger < Test::Unit::TestCase
     }
   end
 
+  def test_bit_count
+    assert_equal(0, 0.bit_count)
+    assert_equal(1, 1.bit_count)
+    assert_equal(1, 2.bit_count)
+    assert_equal(2, 3.bit_count)
+    assert_equal(3, 7.bit_count)
+    assert_equal(3, 0b10101.bit_count)
+    assert_equal(8, 0xff.bit_count)
+    assert_equal(1, 0x100.bit_count)
+
+    # Fixnum boundaries
+    assert_equal(1, (2**30).bit_count)
+    assert_equal(1, (2**62).bit_count)
+
+    # Bignum
+    assert_equal(1, (2**64).bit_count)
+    assert_equal(64, (2**64-1).bit_count)
+    assert_equal(1, (2**1000).bit_count)
+    assert_equal(1000, (2**1000-1).bit_count)
+    assert_equal(2, (2**1000 + 2**500).bit_count)
+
+    0.upto(1000) {|i|
+      n = 2**i
+      assert_equal(1, n.bit_count, "(#{n}).bit_count")
+      assert_equal(i, (n-1).bit_count, "(#{n-1}).bit_count")
+      assert_equal(2, (n+1).bit_count, "(#{n+1}).bit_count") if i >= 1
+    }
+  end
+
+  def test_bit_count_for_negative_numbers
+    assert_raise(ArgumentError) { -1.bit_count }
+    assert_raise(ArgumentError) { -19.bit_count }
+    assert_raise(ArgumentError) { (-2**1000).bit_count }
+  end
+
   def test_digits
     assert_equal([0], 0.digits)
     assert_equal([1], 1.digits)