diff --git a/src/Models/Records/IssuedBy.php b/src/Models/Records/IssuedBy.php
new file mode 100644
index 0000000..ac69723
--- /dev/null
+++ b/src/Models/Records/IssuedBy.php
@@ -0,0 +1,15 @@
+<?php
+namespace Komma\Verifactu\Models\Records;
+
+/**
+ * Factura emitida por un tercero o por el destinatario
+ *
+ * @field EmitidaPorTerceroODestinatario
+ */
+enum IssuedBy: string {
+    /** Emitida por un tercero en nombre del obligado (requiere el bloque Tercero) */
+    case ThirdParty = 'T';
+
+    /** Emitida por el destinatario (autofacturación) */
+    case Recipient = 'D';
+}
diff --git a/src/Models/Records/RegistrationRecord.php b/src/Models/Records/RegistrationRecord.php
index 1301d4b..4681079 100644
--- a/src/Models/Records/RegistrationRecord.php
+++ b/src/Models/Records/RegistrationRecord.php
@@ -90,6 +90,21 @@ class RegistrationRecord extends Record {
     #[Assert\Count(max: 1000)]
     public array $recipients = [];
 
+    /**
+     * Factura emitida por un tercero o por el destinatario (autofacturación)
+     *
+     * @field EmitidaPorTerceroODestinatario
+     */
+    public ?IssuedBy $issuedBy = null;
+
+    /**
+     * Tercero que expide la factura en nombre del obligado
+     *
+     * @field Tercero
+     */
+    #[Assert\Valid]
+    public ?FiscalIdentifier $thirdParty = null;
+
     /**
      * Tipo de factura rectificativa
      *
@@ -252,6 +267,25 @@ private function hasRegimeWithoutTotalCheck(): bool {
         return false;
     }
 
+    #[Assert\Callback]
+    final public function validateIssuedBy(ExecutionContextInterface $context): void {
+        if ($this->issuedBy === IssuedBy::ThirdParty && $this->thirdParty === null) {
+            $context->buildViolation('An invoice issued by a third party requires the third party')
+                ->atPath('thirdParty')
+                ->addViolation();
+        }
+        if ($this->issuedBy !== IssuedBy::ThirdParty && $this->thirdParty !== null) {
+            $context->buildViolation('Only an invoice issued by a third party can carry the third party')
+                ->atPath('thirdParty')
+                ->addViolation();
+        }
+        if ($this->issuedBy === IssuedBy::Recipient && count($this->recipients) === 0) {
+            $context->buildViolation('A self-billed invoice requires its recipient, who issues it')
+                ->atPath('recipients')
+                ->addViolation();
+        }
+    }
+
     #[Assert\Callback]
     final public function validateRecipients(ExecutionContextInterface $context): void {
         if (!isset($this->invoiceType)) {
@@ -418,6 +452,19 @@ protected function importCustomProperties(UXML $recordElement): void {
         }
         $this->description = $description;
 
+        // Issued by a third party or by the recipient
+        $rawIssuedBy = $recordElement->get('sum1:EmitidaPorTerceroODestinatario')?->asText();
+        if ($rawIssuedBy !== null) {
+            $this->issuedBy = IssuedBy::tryFrom($rawIssuedBy) ?? throw new ImportException('Invalid <sum1:EmitidaPorTerceroODestinatario /> value');
+        }
+        $terceroElement = $recordElement->get('sum1:Tercero');
+        if ($terceroElement !== null) {
+            $this->thirdParty = new FiscalIdentifier(
+                $terceroElement->get('sum1:NombreRazon')?->asText() ?? throw new ImportException('Missing <sum1:NombreRazon /> from <sum1:Tercero /> element'),
+                $terceroElement->get('sum1:NIF')?->asText() ?? throw new ImportException('Missing <sum1:NIF /> from <sum1:Tercero /> element'),
+            );
+        }
+
         // Recipients
         foreach ($recordElement->getAll('sum1:Destinatarios/sum1:IDDestinatario') as $destinatarioElement) {
             $recipientName = $destinatarioElement->get('sum1:NombreRazon')?->asText();
@@ -526,6 +573,16 @@ protected function exportCustomProperties(UXML $recordElement): void {
         // Description
         $recordElement->add('sum1:DescripcionOperacion', $this->description);
 
+        // Issued by a third party or by the recipient
+        if ($this->issuedBy !== null) {
+            $recordElement->add('sum1:EmitidaPorTerceroODestinatario', $this->issuedBy->value);
+        }
+        if ($this->thirdParty !== null) {
+            $terceroElement = $recordElement->add('sum1:Tercero');
+            $terceroElement->add('sum1:NombreRazon', $this->thirdParty->name);
+            $terceroElement->add('sum1:NIF', $this->thirdParty->nif);
+        }
+
         // Recipients
         if (count($this->recipients) > 0) {
             $destinatariosElement = $recordElement->add('sum1:Destinatarios');
diff --git a/tests/Models/Records/RegistrationRecordTest.php b/tests/Models/Records/RegistrationRecordTest.php
index 1e789e9..164e9d3 100644
--- a/tests/Models/Records/RegistrationRecordTest.php
+++ b/tests/Models/Records/RegistrationRecordTest.php
@@ -11,6 +11,7 @@
 use Komma\Verifactu\Models\Records\ForeignIdType;
 use Komma\Verifactu\Models\Records\InvoiceIdentifier;
 use Komma\Verifactu\Models\Records\InvoiceType;
+use Komma\Verifactu\Models\Records\IssuedBy;
 use Komma\Verifactu\Models\Records\OperationType;
 use Komma\Verifactu\Models\Records\Record;
 use Komma\Verifactu\Models\Records\RegimeType;
@@ -410,6 +411,54 @@ public function testValidatesReplacedInvoices(): void {
         $record->validate();
     }
 
+    public function testValidatesIssuedBy(): void {
+        $record = Record::fromXml(TestUtils::getXmlFile(__DIR__ . '/registration-record-f1.xml'));
+        $this->assertInstanceOf(RegistrationRecord::class, $record);
+
+        $record->issuedBy = IssuedBy::ThirdParty;
+        try {
+            $record->validate();
+            $this->fail('An invoice issued by a third party without the third party must not validate');
+        } catch (InvalidModelException $e) {
+            $this->assertStringContainsString('requires the third party', $e->getMessage());
+        }
+
+        $record->thirdParty = new FiscalIdentifier('Gestoría Rías SL', 'B70000021');
+        $record->validate();
+
+        $record->issuedBy = IssuedBy::Recipient;
+        try {
+            $record->validate();
+            $this->fail('A self-billed invoice must not carry a third party');
+        } catch (InvalidModelException $e) {
+            $this->assertStringContainsString('Only an invoice issued by a third party', $e->getMessage());
+        }
+
+        $record->thirdParty = null;
+        $record->validate();
+        $this->assertSame(IssuedBy::Recipient, $record->issuedBy);
+    }
+
+    public function testExportsAndImportsIssuedByRecipient(): void {
+        $modelXml = TestUtils::getXmlFile(__DIR__ . '/registration-record-f1.xml');
+        $record = Record::fromXml($modelXml);
+        $this->assertInstanceOf(RegistrationRecord::class, $record);
+        $record->issuedBy = IssuedBy::Recipient;
+
+        $computerSystemXml = $modelXml->get('sum1:SistemaInformatico');
+        $this->assertNotNull($computerSystemXml);
+        $exportedXml = UXML::newInstance('container', null, ['xmlns:sum1' => Record::NS]);
+        $record->export($exportedXml, ComputerSystem::fromXml($computerSystemXml));
+
+        $xml = $exportedXml->get('sum1:RegistroAlta')?->asXML() ?? '';
+        $this->assertStringContainsString('<sum1:EmitidaPorTerceroODestinatario>D</sum1:EmitidaPorTerceroODestinatario>', $xml);
+        $this->assertLessThan(strpos($xml, '<sum1:Destinatarios>'), strpos($xml, '<sum1:EmitidaPorTerceroODestinatario>'));
+
+        $reimported = Record::fromXml($exportedXml->get('sum1:RegistroAlta'));
+        $this->assertInstanceOf(RegistrationRecord::class, $reimported);
+        $this->assertSame(IssuedBy::Recipient, $reimported->issuedBy);
+    }
+
     #[DataProvider('xmlPathsProvider')]
     public function testImportsAndExportsXmlElement(string $xmlPath): void {
         // Import model