diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index ed8fc28b4dda..348205e956d6 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Deprecate using `insert_all`/`upsert_all` with unpersisted records in associations.
+
+    Using these methods on associations containing unpersisted records will now
+    show a deprecation warning, as the unpersisted records will be lost after
+    the operation. This will become an error in Rails 8.2.
+
+    *Nick Schwaderer*
+
 *   Serialized attributes can now be marked as comparable.
 
     A not rare issue when working with serialized attributes is that the serialized representation of an object
diff --git a/activerecord/lib/active_record/associations/collection_proxy.rb b/activerecord/lib/active_record/associations/collection_proxy.rb
index 930179ed0ecd..65127ef1cce7 100644
--- a/activerecord/lib/active_record/associations/collection_proxy.rb
+++ b/activerecord/lib/active_record/associations/collection_proxy.rb
@@ -1128,7 +1128,18 @@ def pretty_print(pp) # :nodoc:
       %w(insert insert_all insert! insert_all! upsert upsert_all).each do |method|
         class_eval <<~RUBY, __FILE__, __LINE__ + 1
           def #{method}(...)
-            scope.#{method}(...).tap { reset }
+            if @association&.target&.any? { |r| r.new_record? }
+              association_name = @association.reflection.name
+              ActiveRecord.deprecator.warn(<<~MSG)
+                Using #{method} on association \#{association_name} with unpersisted records
+                is deprecated. The unpersisted records will be lost after this operation.
+                Please either persist your records first or store them separately before
+                calling #{method}.
+              MSG
+              scope.#{method}(...)
+            else
+              scope.#{method}(...).tap { reset }
+            end
           end
         RUBY
       end
diff --git a/activerecord/test/cases/insert_all_test.rb b/activerecord/test/cases/insert_all_test.rb
index fb39dce3d1f3..c102d6967045 100644
--- a/activerecord/test/cases/insert_all_test.rb
+++ b/activerecord/test/cases/insert_all_test.rb
@@ -863,6 +863,106 @@ def test_insert_all_when_table_name_contains_database
     end
   end
 
+  def test_insert_all_with_unpersisted_records_triggers_deprecation
+    author = Author.create!(name: "Rafael")
+    author.books.build(title: "Unpersisted Book")
+
+    assert_deprecated(ActiveRecord.deprecator) do
+      author.books.insert({ title: "New Book" })
+    end
+
+    author.books.load
+    assert_includes author.books.pluck(:title), "Unpersisted Book"
+  end
+
+  def test_insert_all_without_unpersisted_records_has_no_deprecation
+    author = Author.create!(name: "Rafael")
+
+    assert_not_deprecated(ActiveRecord.deprecator) do
+      author.books.insert_all([{ title: "New Book" }])
+    end
+  end
+
+  def test_insert_with_unpersisted_records_triggers_deprecation
+    author = Author.create!(name: "Rafael")
+    author.books.build(title: "Unpersisted Book")
+
+    assert_deprecated(ActiveRecord.deprecator) do
+      author.books.insert({ title: "New Book" })
+    end
+
+    author.books.load
+    assert_includes author.books.pluck(:title), "Unpersisted Book"
+  end
+
+  def test_insert_without_unpersisted_records_has_no_deprecation
+    author = Author.create!(name: "Rafael")
+
+    assert_not_deprecated(ActiveRecord.deprecator) do
+      author.books.insert({ title: "New Book" })
+    end
+  end
+
+  def test_insert_all_bang_with_unpersisted_record_triggers_deprecation
+    author = Author.create!(name: "Rafael")
+    author.books.build(title: "Unpersisted Book")
+
+    assert_deprecated(ActiveRecord.deprecator) do
+      author.books.insert_all!([{ title: "New Book" }])
+    end
+
+    author.books.load
+    assert_includes author.books.pluck(:title), "Unpersisted Book"
+  end
+
+  def test_insert_all_bang_without_unpersisted_records_has_no_deprecation
+    author = Author.create!(name: "Rafael")
+
+    assert_not_deprecated(ActiveRecord.deprecator) do
+      author.books.insert_all!([{ title: "New Book" }])
+    end
+  end
+
+  def test_upsert_all_with_unpersisted_record_triggers_deprecation
+    author = Author.create!(name: "Rafael")
+    author.books.build(title: "Unpersisted Book")
+
+    assert_deprecated(ActiveRecord.deprecator) do
+      author.books.upsert_all([{ title: "New Book" }])
+    end
+
+    author.books.load
+    assert_includes author.books.pluck(:title), "Unpersisted Book"
+  end
+
+  def test_upsert_all_without_unpersisted_records_has_no_deprecation
+    author = Author.create!(name: "Rafael")
+
+    assert_not_deprecated(ActiveRecord.deprecator) do
+      author.books.upsert_all([{ title: "New Book" }])
+    end
+  end
+
+  def test_upsert_with_unpersisted_record_triggers_deprecation
+    author = Author.create!(name: "Rafael")
+    author.books.build(title: "Unpersisted Book")
+
+    assert_deprecated(ActiveRecord.deprecator) do
+      author.books.upsert({ title: "New Book" })
+    end
+
+    author.books.load
+    assert_includes author.books.pluck(:title), "Unpersisted Book"
+  end
+
+  def test_upsert_without_unpersisted_records_has_no_deprecation
+    author = Author.create!(name: "Rafael")
+
+    assert_not_deprecated(ActiveRecord.deprecator) do
+      author.books.upsert({ title: "New Book" })
+    end
+  end
+
   private
     def capture_log_output
       output = StringIO.new