diff --git a/cpp/src/arrow/array/array_dict.cc b/cpp/src/arrow/array/array_dict.cc
index 2e54e6ec4904..25bf381bbb6a 100644
--- a/cpp/src/arrow/array/array_dict.cc
+++ b/cpp/src/arrow/array/array_dict.cc
@@ -108,10 +108,9 @@ DictionaryArray::DictionaryArray(const std::shared_ptr<DataType>& type,
 }
 
 const std::shared_ptr<Array>& DictionaryArray::dictionary() const {
-  if (!dictionary_) {
-    // TODO(GH-36503) this isn't thread safe
-    dictionary_ = MakeArray(data_->dictionary);
-  }
+  std::call_once(dict_init_flag_, [this]() {
+    const_cast<DictionaryArray*>(this)->dictionary_ = MakeArray(data_->dictionary);
+  });
   return dictionary_;
 }
 
diff --git a/cpp/src/arrow/array/array_dict.h b/cpp/src/arrow/array/array_dict.h
index bf376b51f8c9..8d8e1bfe69a3 100644
--- a/cpp/src/arrow/array/array_dict.h
+++ b/cpp/src/arrow/array/array_dict.h
@@ -19,6 +19,7 @@
 
 #include <cstdint>
 #include <memory>
+#include <mutex>
 
 #include "arrow/array/array_base.h"
 #include "arrow/array/data.h"
@@ -120,6 +121,7 @@ class ARROW_EXPORT DictionaryArray : public Array {
 
   // Lazily initialized when invoking dictionary()
   mutable std::shared_ptr<Array> dictionary_;
+  mutable std::once_flag dict_init_flag_;
 };
 
 /// \brief Helper class for incremental dictionary unification