diff --git a/lib/elixlsx/workbook.ex b/lib/elixlsx/workbook.ex
index f09d557..ab675b1 100644
--- a/lib/elixlsx/workbook.ex
+++ b/lib/elixlsx/workbook.ex
@@ -6,15 +6,18 @@ defmodule Elixlsx.Workbook do
 
   The datetime property can optionally be set to override
   the "created at" date. It defaults to the current time.
+
+  The defined_names property is a map of defined names in the workbook.
   """
   alias Elixlsx.Sheet
   alias Elixlsx.Workbook
 
-  defstruct sheets: [], datetime: nil
+  defstruct sheets: [], datetime: nil, defined_names: %{}
 
   @type t :: %Workbook{
           sheets: nonempty_list(Sheet.t()),
-          datetime: String.t() | integer | nil
+          datetime: String.t() | integer | nil,
+          defined_names: %{String.t() => String.t()}
         }
 
   @doc "Append a sheet at the end."
@@ -30,4 +33,12 @@ defmodule Elixlsx.Workbook do
   def insert_sheet(workbook, sheet, at \\ 0) do
     update_in(workbook.sheets, &List.insert_at(&1, at, sheet))
   end
+
+  @doc """
+  Add a defined name to the workbook.
+  """
+  @spec add_defined_name(Workbook.t(), String.t(), String.t()) :: Workbook.t()
+  def add_defined_name(workbook, name, definition) do
+    update_in(workbook.defined_names, &Map.put(&1, name, definition))
+  end
 end
diff --git a/lib/elixlsx/xml_templates.ex b/lib/elixlsx/xml_templates.ex
index d3dffd8..70fcc96 100644
--- a/lib/elixlsx/xml_templates.ex
+++ b/lib/elixlsx/xml_templates.ex
@@ -775,6 +775,24 @@ defmodule Elixlsx.XMLTemplates do
     |> Enum.map_join(&make_xl_workbook_xml_sheet_entry/1)
   end
 
+  defp workbook_defined_names(%{}) do
+    ""
+  end
+
+  defp workbook_defined_names(defined_names) do
+    """
+    <definedNames>
+    #{Enum.map_join(defined_names, &make_defined_name/1)}
+    </definedNames>
+    """
+  end
+
+  defp make_defined_name({name, definition}) do
+    """
+    <definedName name="#{xml_escape(name)}">#{xml_escape(definition)}</definedName>
+    """
+  end
+
   @doc ~S"""
   Return the data for /xl/workbook.xml
   """
@@ -791,6 +809,9 @@ defmodule Elixlsx.XMLTemplates do
       workbook_sheet_entries(data.sheets, sci) <>
       ~S"""
       </sheets>
+      """ <>
+      workbook_defined_names(data.defined_names) <>
+      """
       <calcPr fullCalcOnLoad="1" iterateCount="100" refMode="A1" iterate="false" iterateDelta="0.001"/>
       </workbook>
       """