diff --git a/lib/toon.rb b/lib/toon.rb
index fc2969b..135a80e 100644
--- a/lib/toon.rb
+++ b/lib/toon.rb
@@ -17,17 +17,19 @@ module Toon
   # @param delimiter [String] Delimiter for array values and tabular rows (default: ',')
   # @param length_marker [String, false] Optional marker to prefix array lengths (default: false)
   # @return [String] TOON-formatted string
-  def encode(input, indent: 2, delimiter: DEFAULT_DELIMITER, length_marker: false)
+  # @return [nil] if writing to output
+  def encode(input, indent: 2, delimiter: DEFAULT_DELIMITER, length_marker: false, output: nil)
     normalized_value = Normalizer.normalize_value(input)
-    options = resolve_options(indent: indent, delimiter: delimiter, length_marker: length_marker)
+    options = resolve_options(indent: indent, delimiter: delimiter, length_marker: length_marker, output:)
     Encoders.encode_value(normalized_value, options)
   end
 
-  def resolve_options(indent:, delimiter:, length_marker:)
+  def resolve_options(indent:, delimiter:, length_marker:, output:)
     {
       indent: indent,
       delimiter: delimiter,
-      length_marker: length_marker
+      length_marker: length_marker,
+      output: output
     }
   end
 end
diff --git a/lib/toon/encoders.rb b/lib/toon/encoders.rb
index 672373f..71fdcda 100644
--- a/lib/toon/encoders.rb
+++ b/lib/toon/encoders.rb
@@ -16,7 +16,7 @@ def encode_value(value, options)
         return Primitives.encode_primitive(value, options[:delimiter])
       end
 
-      writer = LineWriter.new(options[:indent])
+      writer = LineWriter.new(options[:indent], options[:output])
 
       if Normalizer.json_array?(value)
         encode_array(nil, value, writer, 0, options)
@@ -24,6 +24,8 @@ def encode_value(value, options)
         encode_object(value, writer, 0, options)
       end
 
+      options[:output] and return
+
       writer.to_s
     end
 
diff --git a/lib/toon/writer.rb b/lib/toon/writer.rb
index 26db038..83d7806 100644
--- a/lib/toon/writer.rb
+++ b/lib/toon/writer.rb
@@ -1,19 +1,45 @@
 # frozen_string_literal: true
 
 module Toon
+  # LineWriter is a utility class for building formatted output lines with
+  # consistent indentation.
+  #
+  # It manages the construction of output strings by handling indentation and
+  # line termination.
+  #
+  # @api private
   class LineWriter
-    def initialize(indent_size)
-      @lines = []
+    # Initializes a new LineWriter instance with the specified indentation size
+    # and optional output target.
+    #
+    # @param indent_size [ Integer ] the number of spaces to use for each
+    #   indentation level @param output [ StringIO, nil ] the output target to
+    # write to, or nil to use a new StringIO instance
+    def initialize(indent_size, output = nil)
+      @output = output || StringIO.new
       @indentation_string = ' ' * indent_size
+      @started = false
     end
 
+    # Pushes a content line with specified indentation to the output.
+    #
+    # @param depth [ Integer ] the indentation depth level
+    # @param content [ String ] the content to push
+    # @return [ LineWriter ] returns self to allow method chaining
     def push(depth, content)
       indent = @indentation_string * depth
-      @lines << indent + content
+      @output << ?\n if @started
+      @output << indent + content
+      @started = true
+      self
     end
 
+    # Returns the string representation of the written content with trailing
+    # newlines removed.
+    #
+    # @return [ String ] the final output string with trailing newline stripped
     def to_s
-      @lines.join("\n")
+      @output.string
     end
   end
   private_constant :LineWriter
diff --git a/spec/toon_spec.rb b/spec/toon_spec.rb
index 4dfb1de..aefe16e 100644
--- a/spec/toon_spec.rb
+++ b/spec/toon_spec.rb
@@ -771,4 +771,20 @@
       expect(Toon.encode(obj)).to eq('tags[3]: reading,gaming,coding')
     end
   end
+
+  describe 'output option' do
+    let :obj do
+      { 'hello' => '世界' }
+    end
+
+    it 'can encode lines to IO interface object' do
+      output = StringIO.new
+      expect(Toon.encode(obj, output:)).to be_nil
+      expect(output.string).to eq "hello: 世界"
+    end
+
+    it 'can encode with return value' do
+      expect(Toon.encode(obj)).to eq 'hello: 世界'
+    end
+  end
 end