diff --git a/config/feature_flags/development/diff_line_syntax_highlighting.yml b/config/feature_flags/development/diff_line_syntax_highlighting.yml
new file mode 100644
index 0000000000000000000000000000000000000000..3244dad6c24108e95d1f84fa50ff1bc52428bddd
--- /dev/null
+++ b/config/feature_flags/development/diff_line_syntax_highlighting.yml
@@ -0,0 +1,8 @@
+---
+name: diff_line_syntax_highlighting
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56108
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/324159
+milestone: '13.10'
+type: development
+group: group::source code
+default_enabled: false
diff --git a/lib/gitlab/diff/highlight.rb b/lib/gitlab/diff/highlight.rb
index 23859e2573eb36aa14a5d3945358fa55704ea74c..8385bbbb3ded598d1fe42b9b25ff075e85503138 100644
--- a/lib/gitlab/diff/highlight.rb
+++ b/lib/gitlab/diff/highlight.rb
@@ -86,6 +86,41 @@ def apply_marker_ranges_highlight(diff_line, rich_line, index)
       def highlight_line(diff_line)
         return unless diff_file && diff_file.diff_refs
 
+        if Feature.enabled?(:diff_line_syntax_highlighting, project, default_enabled: :yaml)
+          diff_line_highlighting(diff_line)
+        else
+          blob_highlighting(diff_line)
+        end
+      end
+
+      def diff_line_highlighting(diff_line)
+        rich_line = syntax_highlighter(diff_line).highlight(
+          diff_line.text(prefix: false),
+          context: { line_number: diff_line.line }
+        )&.html_safe
+
+        # Only update text if line is found. This will prevent
+        # issues with submodules given the line only exists in diff content.
+        if rich_line
+          line_prefix = diff_line.text =~ /\A(.)/ ? Regexp.last_match(1) : ' '
+          rich_line.prepend(line_prefix).concat("\n")
+        end
+      end
+
+      def syntax_highlighter(diff_line)
+        path = diff_line.removed? ? diff_file.old_path : diff_file.new_path
+
+        @syntax_highlighter ||= {}
+        @syntax_highlighter[path] ||= Gitlab::Highlight.new(
+          path,
+          @raw_lines,
+          language: repository&.gitattribute(path, 'gitlab-language')
+        )
+      end
+
+      # Deprecated: https://gitlab.com/gitlab-org/gitlab/-/issues/324159
+      # ------------------------------------------------------------------------
+      def blob_highlighting(diff_line)
         rich_line =
           if diff_line.unchanged? || diff_line.added?
             new_lines[diff_line.new_pos - 1]&.html_safe
@@ -102,6 +137,7 @@ def highlight_line(diff_line)
       end
 
       # Deprecated: https://gitlab.com/gitlab-org/gitlab/-/issues/324638
+      # ------------------------------------------------------------------------
       def inline_diffs
         @inline_diffs ||= InlineDiff.for_lines(@raw_lines)
       end
diff --git a/lib/gitlab/diff/highlight_cache.rb b/lib/gitlab/diff/highlight_cache.rb
index 2192582348c8fc95d46fcce8c0f0115166f6be08..209462fd6e9dd007e726169f49566ec4e59ed14d 100644
--- a/lib/gitlab/diff/highlight_cache.rb
+++ b/lib/gitlab/diff/highlight_cache.rb
@@ -71,10 +71,12 @@ def key
         strong_memoize(:redis_key) do
           [
             'highlighted-diff-files',
-            diffable.cache_key, VERSION,
+            diffable.cache_key,
+            VERSION,
             diff_options,
             Feature.enabled?(:introduce_marker_ranges, diffable.project, default_enabled: :yaml),
-            Feature.enabled?(:use_marker_ranges, diffable.project, default_enabled: :yaml)
+            Feature.enabled?(:use_marker_ranges, diffable.project, default_enabled: :yaml),
+            Feature.enabled?(:diff_line_syntax_highlighting, diffable.project, default_enabled: :yaml)
           ].join(":")
         end
       end
diff --git a/lib/gitlab/diff/line.rb b/lib/gitlab/diff/line.rb
index 444928b4310f551375e7aa655863f9d67b231656..66f506ec3aa1f8f0a447c481fe018b6065efd7cd 100644
--- a/lib/gitlab/diff/line.rb
+++ b/lib/gitlab/diff/line.rb
@@ -9,8 +9,8 @@ class Line
       SERIALIZE_KEYS = %i(line_code rich_text text type index old_pos new_pos).freeze
 
       attr_reader :line_code, :marker_ranges
-      attr_writer :rich_text
-      attr_accessor :text, :index, :type, :old_pos, :new_pos
+      attr_writer :text, :rich_text
+      attr_accessor :index, :type, :old_pos, :new_pos
 
       def initialize(text, type, index, old_pos, new_pos, parent_file: nil, line_code: nil, rich_text: nil)
         @text, @type, @index = text, type, index
@@ -54,6 +54,12 @@ def set_marker_ranges(marker_ranges)
         @marker_ranges = marker_ranges
       end
 
+      def text(prefix: true)
+        return @text if prefix
+
+        @text&.slice(1..).to_s
+      end
+
       def old_line
         old_pos unless added? || meta?
       end
diff --git a/lib/gitlab/highlight.rb b/lib/gitlab/highlight.rb
index 40dee0142b984457edde249c92f872b2563a629e..e4527f06eff2efc5b64fb4bbebf50edfbdda7ecb 100644
--- a/lib/gitlab/highlight.rb
+++ b/lib/gitlab/highlight.rb
@@ -20,7 +20,9 @@ def initialize(blob_name, blob_content, language: nil)
       @blob_content = blob_content
     end
 
-    def highlight(text, continue: true, plain: false)
+    def highlight(text, continue: false, plain: false, context: {})
+      @context = context
+
       plain ||= text.length > MAXIMUM_TEXT_HIGHLIGHT_SIZE
 
       highlighted_text = highlight_text(text, continue: continue, plain: plain)
@@ -38,6 +40,8 @@ def lexer
 
     private
 
+    attr_reader :context
+
     def custom_language
       return unless @language
 
@@ -53,13 +57,13 @@ def highlight_text(text, continue: true, plain: false)
     end
 
     def highlight_plain(text)
-      @formatter.format(Rouge::Lexers::PlainText.lex(text)).html_safe
+      @formatter.format(Rouge::Lexers::PlainText.lex(text), context).html_safe
     end
 
     def highlight_rich(text, continue: true)
       tag = lexer.tag
       tokens = lexer.lex(text, continue: continue)
-      Timeout.timeout(timeout_time) { @formatter.format(tokens, tag: tag).html_safe }
+      Timeout.timeout(timeout_time) { @formatter.format(tokens, context.merge(tag: tag)).html_safe }
     rescue Timeout::Error => e
       Gitlab::ErrorTracking.track_and_raise_for_dev_exception(e)
       highlight_plain(text)
diff --git a/lib/rouge/formatters/html_gitlab.rb b/lib/rouge/formatters/html_gitlab.rb
index 8f18d6433e00bfd83e22c494512d46b79fa616c4..e0e9677fac7d80312c6a9eaaf1eaa7ea045d62a1 100644
--- a/lib/rouge/formatters/html_gitlab.rb
+++ b/lib/rouge/formatters/html_gitlab.rb
@@ -7,10 +7,11 @@ class HTMLGitlab < Rouge::Formatters::HTML
 
       # Creates a new <tt>Rouge::Formatter::HTMLGitlab</tt> instance.
       #
-      # [+tag+]     The tag (language) of the lexer used to generate the formatted tokens
+      # [+tag+]          The tag (language) of the lexer used to generate the formatted tokens
+      # [+line_number+]  The line number used to populate line IDs
       def initialize(options = {})
-        @line_number = 1
         @tag = options[:tag]
+        @line_number = options[:line_number] || 1
       end
 
       def stream(tokens)
diff --git a/spec/lib/gitlab/diff/highlight_cache_spec.rb b/spec/lib/gitlab/diff/highlight_cache_spec.rb
index 8d29b001f8d5b7a012045cdb313087693e8ff15f..4c56911e6652188b6aa6ad19cbb590bdbddd5768 100644
--- a/spec/lib/gitlab/diff/highlight_cache_spec.rb
+++ b/spec/lib/gitlab/diff/highlight_cache_spec.rb
@@ -238,26 +238,36 @@
     subject { cache.key }
 
     it 'returns cache key' do
-      is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:true:true")
+      is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:true:true:true")
     end
 
-    context 'when feature flag is disabled' do
+    context 'when the `introduce_marker_ranges` feature flag is disabled' do
       before do
         stub_feature_flags(introduce_marker_ranges: false)
       end
 
       it 'returns the original version of the cache' do
-        is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:false:true")
+        is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:false:true:true")
       end
     end
 
-    context 'when use marker ranges feature flag is disabled' do
+    context 'when the `use_marker_ranges` feature flag is disabled' do
       before do
         stub_feature_flags(use_marker_ranges: false)
       end
 
       it 'returns the original version of the cache' do
-        is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:true:false")
+        is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:true:false:true")
+      end
+    end
+
+    context 'when the `diff_line_syntax_highlighting` feature flag is disabled' do
+      before do
+        stub_feature_flags(diff_line_syntax_highlighting: false)
+      end
+
+      it 'returns the original version of the cache' do
+        is_expected.to eq("highlighted-diff-files:#{cache.diffable.cache_key}:2:#{cache.diff_options}:true:true:false")
       end
     end
   end
diff --git a/spec/lib/gitlab/diff/line_spec.rb b/spec/lib/gitlab/diff/line_spec.rb
index a40cd99f6f86e8f30dafc2d55ed659a5e736c341..949def599ae6a3474eaaef6316e2438607fd4abc 100644
--- a/spec/lib/gitlab/diff/line_spec.rb
+++ b/spec/lib/gitlab/diff/line_spec.rb
@@ -45,6 +45,29 @@
     end
   end
 
+  describe '#text' do
+    let(:line) { described_class.new(raw_diff, 'new', 0, 0, 0) }
+    let(:raw_diff) { '+Hello' }
+
+    it 'returns raw diff text' do
+      expect(line.text).to eq('+Hello')
+    end
+
+    context 'when prefix is disabled' do
+      it 'returns raw diff text without prefix' do
+        expect(line.text(prefix: false)).to eq('Hello')
+      end
+
+      context 'when diff is empty' do
+        let(:raw_diff) { '' }
+
+        it 'returns an empty raw diff' do
+          expect(line.text(prefix: false)).to eq('')
+        end
+      end
+    end
+  end
+
   context "when setting rich text" do
     it 'escapes any HTML special characters in the diff chunk header' do
       subject = described_class.new("<input>", "", 0, 0, 0)
diff --git a/spec/lib/gitlab/highlight_spec.rb b/spec/lib/gitlab/highlight_spec.rb
index 9271b868e360737aa3097d03e75894a9bc7d9c42..1a929373716acd37c6ddd18ece113b20c10457b6 100644
--- a/spec/lib/gitlab/highlight_spec.rb
+++ b/spec/lib/gitlab/highlight_spec.rb
@@ -79,6 +79,21 @@ def test(input):
 
         expect(result).to eq(expected)
       end
+
+      context 'when start line number is set' do
+        let(:expected) do
+          %q(<span id="LC10" class="line" lang="diff"><span class="gi">+aaa</span></span>
+<span id="LC11" class="line" lang="diff"><span class="gi">+bbb</span></span>
+<span id="LC12" class="line" lang="diff"><span class="gd">- ccc</span></span>
+<span id="LC13" class="line" lang="diff"> ddd</span>)
+        end
+
+        it 'highlights each line properly' do
+          result = described_class.new(file_name, content).highlight(content, context: { line_number: 10 })
+
+          expect(result).to eq(expected)
+        end
+      end
     end
 
     describe 'with CRLF' do
diff --git a/spec/lib/rouge/formatters/html_gitlab_spec.rb b/spec/lib/rouge/formatters/html_gitlab_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..d45c8c2a8c5674e42d02f98c77b0051bbc2088dd
--- /dev/null
+++ b/spec/lib/rouge/formatters/html_gitlab_spec.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Rouge::Formatters::HTMLGitlab do
+  describe '#format' do
+    subject { described_class.format(tokens, options) }
+
+    let(:lang) { 'ruby' }
+    let(:lexer) { Rouge::Lexer.find_fancy(lang) }
+    let(:tokens) { lexer.lex("def hello", continue: false) }
+    let(:options) { { tag: lang } }
+
+    it 'returns highlighted ruby code' do
+      code = %q{<span id="LC1" class="line" lang="ruby"><span class="k">def</span> <span class="nf">hello</span></span>}
+
+      is_expected.to eq(code)
+    end
+
+    context 'when options are empty' do
+      let(:options) { {} }
+
+      it 'returns highlighted code without language' do
+        code = %q{<span id="LC1" class="line" lang=""><span class="k">def</span> <span class="nf">hello</span></span>}
+
+        is_expected.to eq(code)
+      end
+    end
+
+    context 'when line number is provided' do
+      let(:options) { { tag: lang, line_number: 10 } }
+
+      it 'returns highlighted ruby code with correct line number' do
+        code = %q{<span id="LC10" class="line" lang="ruby"><span class="k">def</span> <span class="nf">hello</span></span>}
+
+        is_expected.to eq(code)
+      end
+    end
+  end
+end