diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8e33dad..a5eae49 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,6 @@
 ## Unreleased
+### Features
+- Add support for tmuxinator start --append
 
 ## 3.3.3
 ### Features
diff --git a/README.md b/README.md
index 121be4f..095c037 100644
--- a/README.md
+++ b/README.md
@@ -398,6 +398,11 @@ Shows tmuxinator's version.
 tmuxinator version
 ```
 
+Append a project's windows to the current session (instead of creating a new session)
+```
+tmuxinator start [project] --append
+```
+
 ## Project Configuration Location
 
 Using environment variables, it's possible to define which directory
diff --git a/lib/tmuxinator/assets/template.erb b/lib/tmuxinator/assets/template.erb
index f0c770e..f8a17b9 100644
--- a/lib/tmuxinator/assets/template.erb
+++ b/lib/tmuxinator/assets/template.erb
@@ -1,17 +1,19 @@
 #!<%= ENV["SHELL"] || "/bin/bash" %>
 
-# Clear rbenv variables before starting tmux
-unset RBENV_VERSION
-unset RBENV_DIR
+<%- if !append? -%>
+  # Clear rbenv variables before starting tmux
+  unset RBENV_VERSION
+  unset RBENV_DIR
 
-<%= tmux %> start-server;
+  <%= tmux %> start-server;
+<%- end -%>
 
 cd <%= root || "." %>
 
 # Run on_project_start command.
 <%= hook_on_project_start %>
 
-<%- if !tmux_has_session? name -%>
+<%- if append? || !tmux_has_session?(name) -%>
 
   # Run pre command.
   <%= pre %>
@@ -98,7 +100,7 @@ cd <%= root || "." %>
   <%= hook_on_project_restart %>
 <%- end -%>
 
-<%- if attach? -%>
+<%- if attach? && !append? -%>
   if [ -z "$TMUX" ]; then
     <%= tmux %> -u attach-session -t <%= name %>
   else
diff --git a/lib/tmuxinator/cli.rb b/lib/tmuxinator/cli.rb
index c26cf43..b7794db 100644
--- a/lib/tmuxinator/cli.rb
+++ b/lib/tmuxinator/cli.rb
@@ -190,7 +190,8 @@ module Tmuxinator
           force_attach: attach,
           force_detach: detach,
           name: project_options[:name],
-          project_config: project_options[:project_config]
+          project_config: project_options[:project_config],
+          append: project_options[:append]
         }
 
         begin
@@ -227,6 +228,24 @@ module Tmuxinator
       def kill_project(project)
         Kernel.exec(project.kill)
       end
+
+      def start_params(name = nil, *args)
+        # project-config takes precedence over a named project in the case that
+        # both are provided.
+        if options["project-config"]
+          args.unshift name if name
+          name = nil
+        end
+
+        {
+          args: args,
+          attach: options[:attach],
+          custom_name: options[:name],
+          name: name,
+          project_config: options["project-config"],
+          append: options["append"],
+        }
+      end
     end
 
     desc "start [PROJECT] [ARGS]", COMMANDS[:start]
@@ -240,22 +259,11 @@ module Tmuxinator
                                     desc: "Path to project config file"
     method_option "suppress-tmux-version-warning",
                   desc: "Don't show a warning for unsupported tmux versions"
-
+    method_option :append, type: :boolean,
+                           desc: "Appends the project windows and panes in " \
+                                 "the current session"
     def start(name = nil, *args)
-      # project-config takes precedence over a named project in the case that
-      # both are provided.
-      if options["project-config"]
-        args.unshift name if name
-        name = nil
-      end
-
-      params = {
-        args: args,
-        attach: options[:attach],
-        custom_name: options[:name],
-        name: name,
-        project_config: options["project-config"]
-      }
+      params = start_params(name, *args)
 
       show_version_warning if version_warning?(
         options["suppress-tmux-version-warning"]
@@ -312,24 +320,15 @@ module Tmuxinator
                          desc: "Give the session a different name"
     method_option "project-config", aliases: "-p",
                                     desc: "Path to project config file"
+    method_option :append, type: :boolean,
+                           desc: "Appends the project windows and panes in " \
+                                 "the current session"
 
     def debug(name = nil, *args)
-      # project-config takes precedence over a named project in the case that
-      # both are provided.
-      if options["project-config"]
-        args.unshift name if name
-        name = nil
-      end
-
-      params = {
-        args: args,
-        attach: options[:attach],
-        custom_name: options[:name],
-        name: name,
-        project_config: options["project-config"]
-      }
+      params = start_params(name, *args)
 
       project = create_project(params)
+
       say project.render
     end
 
diff --git a/lib/tmuxinator/project.rb b/lib/tmuxinator/project.rb
index 7533fe0..dfecd90 100644
--- a/lib/tmuxinator/project.rb
+++ b/lib/tmuxinator/project.rb
@@ -87,13 +87,23 @@ module Tmuxinator
 
       @force_attach = options[:force_attach]
       @force_detach = options[:force_detach]
+      @append = options[:append]
 
-      raise "Cannot force_attach and force_detach at the same time" \
-        if @force_attach && @force_detach
+      validate_options
 
       extend Tmuxinator::WemuxSupport if wemux?
     end
 
+    def validate_options
+      if @force_attach && @force_detach
+        raise "Cannot force_attach and force_detach at the same time"
+      end
+
+      if append? && !tmux_has_session?(name)
+        raise "Cannot append to a session that does not exist"
+      end
+    end
+
     def render
       self.class.render_template(Tmuxinator::Config.template, binding)
     end
@@ -120,11 +130,25 @@ module Tmuxinator
       blank?(root) ? nil : File.expand_path(root).shellescape
     end
 
+    def current_session_name
+      `[[ -n "${TMUX+set}" ]] && tmux display-message -p "#S"`.strip
+    end
+
     def name
-      name = custom_name || yaml["project_name"] || yaml["name"]
+      name =
+        if append?
+          current_session_name
+        else
+          custom_name || yaml["project_name"] || yaml["name"]
+        end
+
       blank?(name) ? nil : name.to_s.shellescape
     end
 
+    def append?
+      @append
+    end
+
     def pre
       pre_config = yaml["pre"]
       parsed_parameters(pre_config)
@@ -167,6 +191,8 @@ module Tmuxinator
     end
 
     def tmux_has_session?(name)
+      return false unless name
+
       # Redirect stderr to /dev/null in order to prevent "failed to connect
       # to server: Connection refused" error message and non-zero exit status
       # if no tmux sessions exist.
@@ -208,7 +234,13 @@ module Tmuxinator
       end
     end
 
+    def last_window_index
+      `tmux list-windows -F '#I'`.split.last.to_i
+    end
+
     def base_index
+      return last_window_index + 1 if append?
+
       get_base_index.to_i
     end
 
@@ -241,7 +273,7 @@ module Tmuxinator
     end
 
     def window(index)
-      "#{name}:#{index}"
+      append? ? ":#{index}" : "#{name}:#{index}"
     end
 
     def send_pane_command(cmd, window_index, _pane_index)
@@ -329,6 +361,8 @@ module Tmuxinator
     end
 
     def tmux_new_session_command
+      return if append?
+
       window = windows.first.tmux_window_name_option
       "#{tmux} new-session -d -s #{name} #{window}"
     end
diff --git a/spec/factories/projects.rb b/spec/factories/projects.rb
index 460da04..87b2053 100644
--- a/spec/factories/projects.rb
+++ b/spec/factories/projects.rb
@@ -115,4 +115,12 @@ FactoryBot.define do
 
     initialize_with { Tmuxinator::Project.load(file) }
   end
+
+  factory :project_with_append, class: Tmuxinator::Project do
+    transient do
+      file { "spec/fixtures/sample.yml" }
+    end
+
+    initialize_with { Tmuxinator::Project.load(file, append: true) }
+  end
 end
diff --git a/spec/lib/tmuxinator/cli_spec.rb b/spec/lib/tmuxinator/cli_spec.rb
index 8426aba..494b1ca 100644
--- a/spec/lib/tmuxinator/cli_spec.rb
+++ b/spec/lib/tmuxinator/cli_spec.rb
@@ -104,6 +104,17 @@ describe Tmuxinator::Cli do
             expect(instance).to receive(:start).with(*args)
             subject
           end
+
+          context "and the append option is passed" do
+            let(:args) { ["sample", "--append"] }
+
+            it "should call #start" do
+              instance = instance_double(cli)
+              expect(cli).to receive(:new).and_return(instance)
+              expect(instance).to receive(:start).with("sample", "--append")
+              subject
+            end
+          end
         end
 
         context "a thor command" do
diff --git a/spec/lib/tmuxinator/project_spec.rb b/spec/lib/tmuxinator/project_spec.rb
index 4c42ea1..8ef506c 100644
--- a/spec/lib/tmuxinator/project_spec.rb
+++ b/spec/lib/tmuxinator/project_spec.rb
@@ -37,6 +37,10 @@ describe Tmuxinator::Project do
     FactoryBot.build(:project_with_alias)
   end
 
+  let(:project_with_append) do
+    FactoryBot.build(:project_with_append)
+  end
+
   it "should include Hooks" do
     expect(project).to be_kind_of(Tmuxinator::Hooks::Project)
   end
@@ -46,6 +50,21 @@ describe Tmuxinator::Project do
       it "creates an instance" do
         expect(project).to be_a(Tmuxinator::Project)
       end
+
+      it "validates append outside a current session" do
+        Tmuxinator::Project.any_instance.stub(tmux_has_session?: false)
+        expect { project_with_append }.to(
+          raise_error(
+            RuntimeError,
+            "Cannot append to a session that does not exist"
+          )
+        )
+      end
+
+      it "validates append within a current session" do
+        Tmuxinator::Project.any_instance.stub(tmux_has_session?: true)
+        expect { project_with_append }.to_not raise_error
+      end
     end
   end
 
@@ -368,6 +387,17 @@ describe Tmuxinator::Project do
         expect(project.base_index).to eq 0
       end
     end
+
+    context "with append set" do
+      before do
+        Tmuxinator::Project.any_instance.stub(tmux_has_session?: true)
+        project_with_append.stub(last_window_index: 3)
+      end
+
+      it "defaults to the next window index" do
+        expect(project_with_append.base_index).to eq 4
+      end
+    end
   end
 
   describe "#startup_window" do
@@ -410,6 +440,13 @@ describe Tmuxinator::Project do
     it "gets the window and index for tmux" do
       expect(project.window(1)).to eq "sample:1"
     end
+
+    context "with append set" do
+      it "excludes the session name" do
+        Tmuxinator::Project.any_instance.stub(tmux_has_session?: true)
+        expect(project_with_append.window(1)).to eq ":1"
+      end
+    end
   end
 
   describe "#name?" do
@@ -576,6 +613,13 @@ describe Tmuxinator::Project do
         expect(project.tmux_new_session_command).to eq command
       end
     end
+
+    context "with append set" do
+      it "returns nothing" do
+        Tmuxinator::Project.any_instance.stub(tmux_has_session?: true)
+        expect(project_with_append.tmux_new_session_command).to be_nil
+      end
+    end
   end
 
   describe "tmux_kill_session_command" do