diff --git a/spec/std/string_spec.cr b/spec/std/string_spec.cr
index 0a3a079f8a11..a67ac15fa6f5 100644
--- a/spec/std/string_spec.cr
+++ b/spec/std/string_spec.cr
@@ -2245,22 +2245,38 @@ describe "String" do
     end
   end
 
-  it "compares non-case insensitive" do
-    "fo".compare("foo").should eq(-1)
-    "foo".compare("fo").should eq(1)
-    "foo".compare("foo").should eq(0)
-    "foo".compare("fox").should eq(-1)
-    "fox".compare("foo").should eq(1)
-    "foo".compare("Foo").should eq(1)
-  end
-
-  it "compares case insensitive" do
-    "fo".compare("FOO", case_insensitive: true).should eq(-1)
-    "foo".compare("FO", case_insensitive: true).should eq(1)
-    "foo".compare("FOO", case_insensitive: true).should eq(0)
-    "foo".compare("FOX", case_insensitive: true).should eq(-1)
-    "fox".compare("FOO", case_insensitive: true).should eq(1)
-    "fo\u{0000}".compare("FO", case_insensitive: true).should eq(1)
+  describe "compare" do
+    it "compares case-sensitive" do
+      "fo".compare("foo").should eq(-1)
+      "foo".compare("fo").should eq(1)
+      "foo".compare("foo").should eq(0)
+      "foo".compare("fox").should eq(-1)
+      "fox".compare("foo").should eq(1)
+      "foo".compare("Foo").should eq(1)
+      "hällo".compare("Hällo").should eq(1)
+      "".compare("").should eq(0)
+    end
+
+    it "compares case-insensitive" do
+      "foo".compare("FO", case_insensitive: true).should eq(1)
+      "FOO".compare("fo", case_insensitive: true).should eq(1)
+      "fo".compare("FOO", case_insensitive: true).should eq(-1)
+      "FOX".compare("foo", case_insensitive: true).should eq(1)
+      "foo".compare("FOX", case_insensitive: true).should eq(-1)
+      "foo".compare("FOO", case_insensitive: true).should eq(0)
+      "hELLo".compare("HellO", case_insensitive: true).should eq(0)
+      "fo\u{0}".compare("FO", case_insensitive: true).should eq(1)
+      "fo".compare("FO\u{0}", case_insensitive: true).should eq(-1)
+      "\u{0}".compare("\u{0}", case_insensitive: true).should eq(0)
+      "z".compare("hello", case_insensitive: true).should eq(1)
+      "h".compare("zzz", case_insensitive: true).should eq(-1)
+      "ä".compare("äA", case_insensitive: true).should eq(-1)
+      "äÄ".compare("äÄ", case_insensitive: true).should eq(0)
+      "heIIo".compare("heııo", case_insensitive: true, options: Unicode::CaseOptions::Turkic).should eq(0)
+      "".compare("abc", case_insensitive: true).should eq(-1)
+      "abc".compare("", case_insensitive: true).should eq(1)
+      "abcA".compare("abca", case_insensitive: true).should eq(0)
+    end
   end
 
   it "builds with write_byte" do
diff --git a/src/string.cr b/src/string.cr
index 8e3c2b854db5..145e082500ec 100644
--- a/src/string.cr
+++ b/src/string.cr
@@ -2410,10 +2410,6 @@ class String
   # this string is less, equal or greater than *other*, optionally in a *case_insensitive*
   # manner.
   #
-  # If *case_insitive* is `false`, this method delegates to `<=>`. Otherwise,
-  # the strings are compared char-by-char, and ASCII characters are compared in a
-  # case-insensitive way.
-  #
   # ```
   # "abcdef".compare("abcde")   # => 1
   # "abcdef".compare("abcdef")  # => 0
@@ -2422,29 +2418,55 @@ class String
   #
   # "abcdef".compare("ABCDEF", case_insensitive: true) # => 0
   # "abcdef".compare("ABCDEG", case_insensitive: true) # => -1
+  #
+  # "heIIo".compare("heııo", case_insensitive: true, Unicode::CaseOptions::Turkic) # => 0
   # ```
-  def compare(other : String, case_insensitive = false)
+  def compare(other : String, case_insensitive = false, options = Unicode::CaseOptions::None)
     return self <=> other unless case_insensitive
 
-    reader1 = Char::Reader.new(self)
-    reader2 = Char::Reader.new(other)
-    ch1 = reader1.current_char
-    ch2 = reader2.current_char
+    if ascii_only? && other.ascii_only?
+      position = 0
 
-    while reader1.has_next? && reader2.has_next?
-      cmp = ch1.downcase <=> ch2.downcase
-      return cmp.sign if cmp != 0
+      while position < bytesize && position < other.bytesize
+        byte1 = to_unsafe[position]
+        byte2 = other.to_unsafe[position]
 
-      ch1 = reader1.next_char
-      ch2 = reader2.next_char
-    end
+        # Lowercase both bytes
+        if 65 <= byte1 <= 90
+          byte1 += 32
+        end
+        if 65 <= byte2 <= 90
+          byte2 += 32
+        end
+
+        comparison = byte1 <=> byte2
+        return comparison unless comparison == 0
+
+        position += 1
+      end
 
-    if reader1.has_next?
-      1
-    elsif reader2.has_next?
-      -1
+      bytesize <=> other.bytesize
     else
-      0
+      reader1 = Char::Reader.new(self)
+      reader2 = Char::Reader.new(other)
+      char1 = reader1.current_char
+      char2 = reader2.current_char
+
+      while reader1.has_next? && reader2.has_next?
+        comparison = char1.downcase(options) <=> char2.downcase(options)
+        return comparison.sign unless comparison == 0
+
+        char1 = reader1.next_char
+        char2 = reader2.next_char
+      end
+
+      if reader1.has_next?
+        1
+      elsif reader2.has_next?
+        -1
+      else
+        0
+      end
     end
   end