diff --git a/app/models/type.rb b/app/models/type.rb
index 0e6ab797ceb0..e7a509ea464b 100644
--- a/app/models/type.rb
+++ b/app/models/type.rb
@@ -62,17 +62,29 @@ def copy_from_type(source_type)
                           join_table: "#{table_name_prefix}custom_fields_types#{table_name_suffix}",
                           association_foreign_key: "custom_field_id"
 
-  acts_as_list
+  belongs_to :parent, class_name: "Type", optional: true
+  has_many :children, class_name: "Type", foreign_key: :parent_id, inverse_of: :parent,
+                      dependent: :restrict_with_error
+
+  acts_as_list scope: :parent_id
 
   validates :name,
             presence: true,
-            uniqueness: { case_sensitive: false },
+            uniqueness: { scope: :parent_id, case_sensitive: false },
             length: { maximum: 255 }
 
+  validate :parent_is_a_root
+  validate :not_own_parent
+  validate :cannot_have_children_when_child
+  validate :standard_type_stays_root
+  validate :parent_frozen_with_work_packages
+
   scopes :milestone
 
   default_scope { order("position ASC") }
 
+  scope :roots, -> { where(parent_id: nil) }
+  scope :subtypes, -> { where.not(parent_id: nil) }
   scope :without_standard, -> { where(is_standard: false).order(:position) }
   scope :default, -> { where(is_default: true) }
   scope :visible, ->(user = User.current) {
@@ -133,6 +145,14 @@ def enabled_in?(object)
     object.types.include?(self)
   end
 
+  def root
+    parent || self
+  end
+
+  def family
+    [root, *root.children]
+  end
+
   def replacement_pattern_defined_for?(attribute)
     enabled_patterns.key?(attribute)
   end
@@ -153,4 +173,30 @@ def check_integrity
 
     true
   end
+
+  def parent_is_a_root
+    return if parent.nil?
+
+    errors.add(:parent, :must_be_a_root) if parent.parent_id.present?
+  end
+
+  def not_own_parent
+    errors.add(:parent, :cannot_be_self) if parent_id.present? && parent_id == id
+  end
+
+  def cannot_have_children_when_child
+    return if parent_id.blank? || new_record?
+
+    errors.add(:parent, :cannot_have_children) if children.exists?
+  end
+
+  def standard_type_stays_root
+    errors.add(:parent, :standard_type_must_be_root) if is_standard? && parent_id.present?
+  end
+
+  def parent_frozen_with_work_packages
+    return unless parent_id_changed? && persisted?
+
+    errors.add(:parent, :cannot_change_with_work_packages) if WorkPackage.exists?(type_id: id)
+  end
 end
diff --git a/config/locales/en.yml b/config/locales/en.yml
index 55eabaed0a7b..af90a6b547fa 100644
--- a/config/locales/en.yml
+++ b/config/locales/en.yml
@@ -2091,10 +2091,12 @@ en:
       type:
         description: "Default text for description"
         attribute_groups: "Form configuration"
+        children: "Sub-types"
         is_in_roadmap: "Displayed in roadmap by default"
         is_default: "Activated for new projects by default"
         is_milestone: "Is milestone"
         color: "Color"
+        parent: "Parent type"
         patterns: "Patterns"
       remote_identity:
         auth_source: "Auth Source"
@@ -2634,6 +2636,12 @@ en:
               duplicate_group: "The group name '%{group}' is used more than once. Group names must be unique."
               query_invalid: "The embedded query '%{group}' is invalid: %{details}"
               group_without_name: "Group name can't be blank."
+            parent:
+              cannot_be_self: "can't be the type itself."
+              cannot_change_with_work_packages: "can't be changed because work packages of this type already exist."
+              cannot_have_children: "can't be set because this type already has sub-types."
+              must_be_a_root: "must itself be a top-level type. Types can only be nested one level deep."
+              standard_type_must_be_root: "can't be set for the standard type."
             patterns:
               invalid_tokens: "One or more attributes inside the field are not valid. Please, fix the attributes before saving."
         user:
diff --git a/db/migrate/20260703151859_add_parent_to_types.rb b/db/migrate/20260703151859_add_parent_to_types.rb
new file mode 100644
index 000000000000..c12d631c3be5
--- /dev/null
+++ b/db/migrate/20260703151859_add_parent_to_types.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+#-- copyright
+# OpenProject is an open source project management software.
+# Copyright (C) the OpenProject GmbH
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License version 3.
+#
+# OpenProject is a fork of ChiliProject, which is a fork of Redmine. The copyright follows:
+# Copyright (C) 2006-2013 Jean-Philippe Lang
+# Copyright (C) 2010-2013 the ChiliProject Team
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+#
+# See COPYRIGHT and LICENSE files for more details.
+#++
+
+class AddParentToTypes < ActiveRecord::Migration[8.1]
+  def up
+    add_reference :types, :parent, foreign_key: { to_table: :types }, null: true
+
+    remove_index :types, name: "index_types_on_LOWER_name"
+
+    add_index :types, "LOWER(name), parent_id",
+              unique: true,
+              nulls_not_distinct: true,
+              name: "index_types_on_LOWER_name_and_parent_id"
+  end
+
+  def down
+    remove_index :types, name: "index_types_on_LOWER_name_and_parent_id"
+    add_index :types, "LOWER(name)", unique: true, name: "index_types_on_LOWER_name"
+
+    remove_reference :types, :parent
+  end
+end
diff --git a/spec/models/type_spec.rb b/spec/models/type_spec.rb
index 703747fa2f5e..d46bb6a2ad89 100644
--- a/spec/models/type_spec.rb
+++ b/spec/models/type_spec.rb
@@ -262,4 +262,122 @@
       end
     end
   end
+
+  describe "hierarchy" do
+    let!(:parent) { create(:type) }
+    let!(:child) { create(:type, parent:) }
+
+    describe "associations" do
+      it "exposes parent and children" do
+        expect(child.parent).to eq(parent)
+        expect(parent.children).to contain_exactly(child)
+      end
+    end
+
+    describe "scopes" do
+      it ".roots returns only top level types" do
+        expect(described_class.roots).to include(parent)
+        expect(described_class.roots).not_to include(child)
+      end
+
+      it ".subtypes returns only nested types" do
+        expect(described_class.subtypes).to contain_exactly(child)
+      end
+    end
+
+    describe "#root" do
+      it "returns the parent for a child" do
+        expect(child.root).to eq(parent)
+      end
+
+      it "returns itself for a root" do
+        expect(parent.root).to eq(parent)
+      end
+    end
+
+    describe "#family" do
+      it "returns the root followed by its children" do
+        expect(child.family).to eq([parent, child])
+        expect(parent.family).to eq([parent, child])
+      end
+    end
+
+    describe "positioning" do
+      it "scopes position per parent so each group is numbered independently" do
+        parent_a = create(:type)
+        parent_b = create(:type)
+        a1 = create(:type, parent: parent_a, position: nil, name: "A1")
+        a2 = create(:type, parent: parent_a, position: nil, name: "A2")
+        b1 = create(:type, parent: parent_b, position: nil, name: "B1")
+
+        expect([a1.position, a2.position]).to eq([1, 2])
+        expect(b1.position).to eq(1)
+      end
+    end
+
+    describe "validations" do
+      it "rejects nesting deeper than one level" do
+        grandchild = build(:type, parent: child)
+
+        expect(grandchild).not_to be_valid
+      end
+
+      it "rejects a type becoming its own parent" do
+        parent.parent = parent
+
+        expect(parent).not_to be_valid
+      end
+
+      it "rejects giving a parent to a type that already has children" do
+        other_root = create(:type)
+        parent.parent = other_root
+
+        expect(parent).not_to be_valid
+      end
+
+      it "rejects the standard type having a parent" do
+        standard = create(:type_standard)
+        standard.parent = parent
+
+        expect(standard).not_to be_valid
+      end
+
+      it "allows the same name under different parents" do
+        other_parent = create(:type)
+        create(:type, parent:, name: "Shared")
+        duplicate = build(:type, parent: other_parent, name: "Shared")
+
+        expect(duplicate).to be_valid
+      end
+
+      it "rejects a duplicate name under the same parent" do
+        create(:type, parent:, name: "Shared")
+        duplicate = build(:type, parent:, name: "Shared")
+
+        expect(duplicate).not_to be_valid
+      end
+
+      it "freezes parent_id once work packages exist" do
+        other_root = create(:type)
+        create(:work_package, type: child)
+
+        child.parent = other_root
+        expect(child).not_to be_valid
+      end
+
+      it "allows editing a type with work packages when the parent is unchanged" do
+        create(:work_package, type: child)
+
+        child.name = "Renamed"
+        expect(child).to be_valid
+      end
+    end
+
+    describe "deletion" do
+      it "is blocked while children exist" do
+        expect(parent.destroy).to be_falsey
+        expect(parent).to be_persisted
+      end
+    end
+  end
 end