diff --git a/modules/site_tools_msc_2020/js/msc-select.js b/modules/site_tools_msc_2020/js/msc-select.js
new file mode 100644
index 0000000..28732dc
--- /dev/null
+++ b/modules/site_tools_msc_2020/js/msc-select.js
@@ -0,0 +1,5 @@
+/**
+ * @file
+ * Reservado para melhorias futuras do widget MSC 2020.
+ * A lógica de cascata entre os selects é gerida via Drupal AJAX (#ajax).
+ */
diff --git a/modules/site_tools_msc_2020/site_tools_msc_2020.libraries.yml b/modules/site_tools_msc_2020/site_tools_msc_2020.libraries.yml
new file mode 100644
index 0000000..5bc43ad
--- /dev/null
+++ b/modules/site_tools_msc_2020/site_tools_msc_2020.libraries.yml
@@ -0,0 +1,2 @@
+msc_term_select_widget:
+  version: VERSION
diff --git a/modules/site_tools_msc_2020/src/Plugin/Field/FieldWidget/MscTermSelectWidget.php b/modules/site_tools_msc_2020/src/Plugin/Field/FieldWidget/MscTermSelectWidget.php
new file mode 100644
index 0000000..3413bc7
--- /dev/null
+++ b/modules/site_tools_msc_2020/src/Plugin/Field/FieldWidget/MscTermSelectWidget.php
@@ -0,0 +1,222 @@
+<?php
+
+namespace Drupal\site_tools_msc_2020\Plugin\Field\FieldWidget;
+
+use Drupal\Component\Utility\Html;
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Field\FieldDefinitionInterface;
+use Drupal\Core\Field\FieldItemListInterface;
+use Drupal\Core\Field\WidgetBase;
+use Drupal\Core\Form\FormStateInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Widget de seleção em cascata para o vocabulário MSC 2020.
+ *
+ * O primeiro select (categorias pai) dispara um AJAX que reconstrói o segundo
+ * select (subcampos) via servidor. O valor armazenado no campo é o TID
+ * selecionado no segundo select: o TID do pai para "área geral", ou o TID do
+ * filho para um subcampo específico.
+ *
+ * @FieldWidget(
+ *   id = "msc_term_select",
+ *   label = @Translation("MSC 2020 — Seleção em cascata"),
+ *   field_types = {
+ *     "entity_reference"
+ *   }
+ * )
+ */
+class MscTermSelectWidget extends WidgetBase {
+
+  public function __construct(
+    $plugin_id,
+    $plugin_definition,
+    FieldDefinitionInterface $field_definition,
+    array $settings,
+    array $third_party_settings,
+    protected EntityTypeManagerInterface $entityTypeManager,
+  ) {
+    parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
+  }
+
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition): static {
+    return new static(
+      $plugin_id,
+      $plugin_definition,
+      $configuration['field_definition'],
+      $configuration['settings'],
+      $configuration['third_party_settings'],
+      $container->get('entity_type.manager'),
+    );
+  }
+
+  public static function isApplicable(FieldDefinitionInterface $field_definition): bool {
+    $storage = $field_definition->getFieldStorageDefinition();
+    if ($storage->getSetting('target_type') !== 'taxonomy_term') {
+      return FALSE;
+    }
+    $handler_settings = $field_definition->getSetting('handler_settings');
+    return !empty($handler_settings['target_bundles']['msc_2020']);
+  }
+
+  public function formElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state): array {
+    $data = $this->buildTermData();
+
+    // getUserInput() contém os dados crus submetidos e NÃO é afetado pelo
+    // #limit_validation_errors do botão "Add another item" (que descarta
+    // valores fora do caminho do campo em $form_state->getValues()).
+    $field_name  = $this->fieldDefinition->getName();
+    $field_parents = $element['#field_parents'] ?? $form['#parents'] ?? [];
+    $base = array_merge($field_parents, [$field_name, $delta]);
+
+    $raw_input = $form_state->getUserInput();
+    $parent_exists = FALSE;
+    $submitted_parent = NestedArray::getValue(
+      $raw_input,
+      array_merge($base, ['parent_select']),
+      $parent_exists,
+    );
+
+    if ($parent_exists) {
+      // Rebuild pós-submissão (AJAX de parent_select, "Add another item", etc.)
+      $initial_parent = $submitted_parent !== '' ? (int) $submitted_parent : NULL;
+      $child_exists = FALSE;
+      $submitted_child = NestedArray::getValue(
+        $raw_input,
+        array_merge($base, ['child_select']),
+        $child_exists,
+      );
+      $initial_child = ($child_exists && $submitted_child !== '' && $submitted_child !== NULL)
+        ? (int) $submitted_child
+        : $initial_parent;
+    }
+    else {
+      // Carga inicial: deriva do valor persistido na entidade.
+      $current_value = $items[$delta]->target_id ? (int) $items[$delta]->target_id : NULL;
+      [$initial_parent, $initial_child] = $this->resolveInitialValues($current_value, $data);
+    }
+
+    // Opções do segundo select: pré-populadas para a categoria pai atual.
+    $child_options = ['' => $this->t('— selecione uma categoria primeiro —')];
+    if ($initial_parent !== NULL) {
+      $child_options = [];
+      $child_options[$initial_parent] = $this->t('— área geral (sem subcampo) —');
+      $child_options += $data['children'][$initial_parent] ?? [];
+    }
+
+    $field_id  = $field_name . '--' . $delta;
+    $wrapper_id = Html::getId('msc-child-' . $field_id);
+
+    $element['parent_select'] = [
+      '#type' => 'select',
+      '#title' => $element['#title'] ?? $this->t('Área MSC 2020'),
+      '#title_display' => $element['#title_display'] ?? 'before',
+      '#options' => ['' => $this->t('— selecione uma categoria —')] + $data['parents'],
+      '#default_value' => $initial_parent ?? '',
+      '#required' => !empty($element['#required']),
+      '#attributes' => ['class' => ['msc-parent-select']],
+      '#ajax' => [
+        'callback' => [static::class, 'rebuildChildSelect'],
+        'wrapper' => $wrapper_id,
+        'event' => 'change',
+      ],
+    ];
+
+    // O child_select é o elemento cujo valor é salvo no campo.
+    // #validated => TRUE: aceita o valor submetido sem verificar #options,
+    // necessário pois as opções podem ter mudado entre submissões.
+    $element['child_select'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Subcampo'),
+      '#prefix' => '<div id="' . $wrapper_id . '">',
+      '#suffix' => '</div>',
+      '#options' => $child_options,
+      '#default_value' => $initial_child ?? '',
+      '#validated' => TRUE,
+      '#attributes' => ['class' => ['msc-child-select']],
+    ];
+
+    return $element;
+  }
+
+  /**
+   * Callback AJAX: reconstrói o child_select quando o parent_select muda.
+   *
+   * Drupal faz o rebuild completo do formulário antes de chamar este método,
+   * então formElement() já calculou as opções corretas para o novo pai.
+   */
+  public static function rebuildChildSelect(array &$form, FormStateInterface $form_state): array {
+    $parents = $form_state->getTriggeringElement()['#array_parents'];
+    array_pop($parents);
+    $parents[] = 'child_select';
+    return NestedArray::getValue($form, $parents);
+  }
+
+  /**
+   * Carrega todos os termos msc_2020 e retorna arrays de pais e filhos.
+   *
+   * @return array{parents: array<int,string>, children: array<int,array<int,string>>}
+   */
+  protected function buildTermData(): array {
+    $tree = $this->entityTypeManager
+      ->getStorage('taxonomy_term')
+      ->loadTree('msc_2020', 0, NULL, TRUE);
+
+    $code_to_tid = [];
+    $parents = [];
+    $children = [];
+
+    foreach ($tree as $term) {
+      $code = $term->get('field_msc_code')->value;
+      $tid = (int) $term->id();
+      $code_to_tid[$code] = $tid;
+      if (strlen($code) === 2) {
+        $parents[$tid] = $code . ' — ' . $term->label();
+      }
+    }
+
+    foreach ($tree as $term) {
+      $code = $term->get('field_msc_code')->value;
+      if (strlen($code) > 2) {
+        $parent_code = substr($code, 0, 2);
+        $parent_tid = $code_to_tid[$parent_code] ?? NULL;
+        if ($parent_tid) {
+          $tid = (int) $term->id();
+          $children[$parent_tid][$tid] = $code . ' — ' . $term->label();
+        }
+      }
+    }
+
+    return ['parents' => $parents, 'children' => $children];
+  }
+
+  /**
+   * Resolve o par (parent_tid, child_tid) a partir do TID persistido.
+   *
+   * @return array{int|null, int|null}
+   */
+  protected function resolveInitialValues(?int $current_value, array $data): array {
+    if ($current_value === NULL) {
+      return [NULL, NULL];
+    }
+    if (isset($data['parents'][$current_value])) {
+      return [$current_value, $current_value];
+    }
+    foreach ($data['children'] as $parent_tid => $kids) {
+      if (isset($kids[$current_value])) {
+        return [$parent_tid, $current_value];
+      }
+    }
+    return [NULL, NULL];
+  }
+
+  public function massageFormValues(array $values, array $form, FormStateInterface $form_state): array {
+    foreach ($values as &$value) {
+      $child = $value['child_select'] ?? '';
+      $value = ['target_id' => $child !== '' ? (int) $child : NULL];
+    }
+    return $values;
+  }
+
+}