diff --git a/search_api_field_map_domain/search_api_field_map_domain.info.yml b/search_api_field_map_domain/search_api_field_map_domain.info.yml
new file mode 100644
index 0000000..89879ae
--- /dev/null
+++ b/search_api_field_map_domain/search_api_field_map_domain.info.yml
@@ -0,0 +1,9 @@
+name: Search API Field Map Domain
+type: module
+description: Allows indexing multiple Drupal sites into a single search index by Domain.
+core: 8.x
+package: Search
+dependencies:
+  - search_api_solr
+  - field
+  - domain
diff --git a/search_api_field_map_domain/src/Plugin/search_api/processor/MappedDomain.php b/search_api_field_map_domain/src/Plugin/search_api/processor/MappedDomain.php
new file mode 100644
index 0000000..299298b
--- /dev/null
+++ b/search_api_field_map_domain/src/Plugin/search_api/processor/MappedDomain.php
@@ -0,0 +1,126 @@
+<?php
+
+namespace Drupal\search_api_field_map_domain\Plugin\search_api\processor;
+
+use Drupal\domain\DomainNegotiator;
+use Drupal\domain\DomainStorage;
+use Drupal\search_api\Datasource\DatasourceInterface;
+use Drupal\search_api\Item\ItemInterface;
+use Drupal\search_api_field_map_domain\Plugin\search_api\processor\Property\MappedDomainProperty;
+use Drupal\search_api\Processor\ProcessorPluginBase;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Create a field that maps domains to site names.
+ *
+ * @see \Drupal\search_api_field_map_domain\Plugin\search_api\processor\Property\MappedDomainProperty
+ *
+ * @SearchApiProcessor(
+ *   id = "mapped_domain",
+ *   label = @Translation("Mapped domain"),
+ *   description = @Translation("Create a field that maps domains to provided values."),
+ *   stages = {
+ *     "add_properties" = 20,
+ *   },
+ *   locked = true,
+ *   hidden = true,
+ * )
+ */
+class MappedDomain extends ProcessorPluginBase {
+  // @var $domainNegotiator DomainNegotiator.
+  private $domainNegotiator;
+
+  // @var $domainStorage DomainStorage.
+  private $domainStorage;
+
+  /**
+   * @param ContainerInterface $container
+   *
+   * @param array $configuration
+   * @param $plugin_id
+   * @param $plugin_definition
+   *
+   * @return MappedDomain
+   */
+  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
+
+    $domain_negotiator = $container->get('domain.negotiator');
+    $domain_storage = $container->get('entity_type.manager')->getStorage('domain');
+
+    return new static (
+      $configuration,
+      $plugin_id,
+      $plugin_definition,
+      $domain_negotiator,
+      $domain_storage
+    );
+  }
+
+  /**
+   * MappedDomain constructor.
+   *
+   * @param array $configuration
+   * @param $plugin_id
+   * @param $plugin_definition
+   * @param DomainNegotiator $domain_negotiator
+   */
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, DomainNegotiator $domain_negotiator, DomainStorage $domain_storage) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
+
+    $this->domainNegotiator = $domain_negotiator;
+    $this->domainStorage = $domain_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getPropertyDefinitions(DatasourceInterface $datasource = NULL) {
+    $properties = [];
+
+    if (!$datasource) {
+      $definition = [
+        'label' => $this->t('Mapped domain'),
+        'description' => $this->t('Create a field that maps domains to provided values.'),
+        'type' => 'string',
+        'processor_id' => $this->getPluginId(),
+      ];
+      $properties['mapped_domain'] = new MappedDomainProperty($definition, $this->domainStorage);
+    }
+
+    return $properties;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function addFieldValues(ItemInterface $item) {
+    // Get all of the mapped fields on our item.
+    $mapped_fields = $this->getFieldsHelper()
+      ->filterForPropertyPath($item->getFields(), NULL, 'mapped_domain');
+
+    // Get the entity object, bail if there's somehow not one.
+    $entity = $item->getOriginalObject()->getValue();
+    if (!$entity) {
+      // Apparently we were active for a wrong item.
+      return;
+    }
+
+    foreach ($mapped_fields as $mapped_field) {
+
+      // Get configuration for the field.
+      $configuration = $mapped_field->getConfiguration();
+
+      // Get the current domain.
+      $domain = $this->domainNegotiator->getActiveId();
+
+      // If there's a config item for the entity and bundle type we're in, set the value for the field.
+      if(!empty($configuration['field_data'][$domain])) {
+        // If the token replacement produces a value, add to this item.
+        $value = $configuration['field_data'][$domain];
+
+        // Do not use setValues(), since that doesn't preprocess the values according to their data type.
+        $mapped_field->addValue($value);
+      }
+    }
+  }
+}
diff --git a/search_api_field_map_domain/src/Plugin/search_api/processor/Property/MappedDomainProperty.php b/search_api_field_map_domain/src/Plugin/search_api/processor/Property/MappedDomainProperty.php
new file mode 100644
index 0000000..a8521af
--- /dev/null
+++ b/search_api_field_map_domain/src/Plugin/search_api/processor/Property/MappedDomainProperty.php
@@ -0,0 +1,83 @@
+<?php
+
+namespace Drupal\search_api_field_map_domain\Plugin\search_api\processor\Property;
+
+use Drupal\domain\DomainStorage;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\search_api\Item\FieldInterface;
+use Drupal\search_api\Processor\ConfigurablePropertyBase;
+
+/**
+ * Defines an "mapped domain" property.
+ *
+ * @see \Drupal\search_api_field_map_domain\Plugin\search_api\processor\MappedDomain
+ */
+class MappedDomainProperty extends ConfigurablePropertyBase {
+
+  use StringTranslationTrait;
+
+  // @var $domainStorage DomainStorage.
+  private $domainStorage;
+
+  public function __construct($definition, DomainStorage $domain_storage) {
+    parent::__construct($definition);
+
+    $this->domainStorage = $domain_storage;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'type' => 'union',
+      'fields' => [],
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildConfigurationForm(FieldInterface $field, array $form, FormStateInterface $form_state) {
+    $index = $field->getIndex();
+    $configuration = $field->getConfiguration();
+
+    $form['#attached']['library'][] = 'search_api/drupal.search_api.admin_css';
+    $form['#tree'] = TRUE;
+
+    $form['field_data'] = [
+      '#type' => 'item',
+      '#title' => $this->t('Mapped data'),
+      '#description' => $this->t('Set the data to be sent to the index for each domain.'),
+    ];
+
+    $domains = $this->domainStorage->loadMultipleSorted(NULL, TRUE);
+
+    foreach ($domains as $domain) {
+        $form['field_data'][$domain->get('id')] = [
+          '#type' => 'textfield',
+          '#title' => $this->t('Field data for %domain', ['%domain' => $domain->get('name')]),
+        ];
+
+        // Set the default value if something already exists in our config.
+        if (isset($configuration['field_data'][$domain->get('id')])) {
+          $form['field_data'][$domain->get('id')]['#default_value'] = $configuration['field_data'][$domain->get('id')];
+        }
+    }
+
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitConfigurationForm(FieldInterface $field, array &$form, FormStateInterface $form_state) {
+    $values = [
+      'field_data' => array_filter($form_state->getValue('field_data')),
+    ];
+
+    $field->setConfiguration($values);
+  }
+
+}