diff --git a/InventoryCatalog/Test/Integration/CompositeParentSalabilityOnNonDefaultStockTest.php b/InventoryCatalog/Test/Integration/CompositeParentSalabilityOnNonDefaultStockTest.php
new file mode 100644
index 000000000000..550c4e0272b1
--- /dev/null
+++ b/InventoryCatalog/Test/Integration/CompositeParentSalabilityOnNonDefaultStockTest.php
@@ -0,0 +1,117 @@
+<?php
+/**
+ * Copyright 2026 Adobe
+ * All Rights Reserved.
+ */
+declare(strict_types=1);
+
+namespace Magento\InventoryCatalog\Test\Integration;
+
+use Magento\InventoryApi\Api\Data\SourceItemInterface;
+use Magento\InventoryApi\Api\Data\SourceItemInterfaceFactory;
+use Magento\InventoryApi\Api\SourceItemsSaveInterface;
+use Magento\InventorySalesApi\Model\GetStockItemDataInterface;
+use Magento\TestFramework\Fixture\DataFixture;
+use Magento\TestFramework\Fixture\DbIsolation;
+use Magento\TestFramework\Helper\Bootstrap;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Regression guard for magento/inventory#3350.
+ *
+ * A composite parent whose children are out of stock at the DEFAULT source but still in stock at another
+ * source must remain salable in the stock those other sources back. The parent's legacy stock item is
+ * default-stock data; recomputing it from default-scope children and then letting it decide another
+ * stock's salability is what #3350 reported.
+ */
+class CompositeParentSalabilityOnNonDefaultStockTest extends TestCase
+{
+    private const NON_DEFAULT_STOCK_ID = 10;
+
+    private const PARENT_SKU = 'configurable_1';
+
+    private const CHILD_SKUS = ['simple_11', 'simple_21', 'simple_31'];
+
+    /**
+     * @var GetStockItemDataInterface
+     */
+    private $getStockItemData;
+
+    protected function setUp(): void
+    {
+        $this->getStockItemData = Bootstrap::getObjectManager()->get(GetStockItemDataInterface::class);
+    }
+
+    /**
+     * @return void
+     */
+    #[
+        DbIsolation(false),
+        DataFixture('Magento_InventoryApi::Test/_files/sources.php'),
+        DataFixture('Magento_InventoryApi::Test/_files/stocks.php'),
+        DataFixture('Magento_InventoryApi::Test/_files/stock_source_links.php'),
+        DataFixture('Magento_InventorySalesApi::Test/_files/websites_with_stores.php'),
+        DataFixture('Magento_InventorySalesApi::Test/_files/stock_website_sales_channels.php'),
+        DataFixture('Magento/ConfigurableProduct/_files/configurable_attribute.php'),
+        DataFixture('Magento_InventoryConfigurableProductIndexer::Test/_files/product_configurable_multiple.php'),
+        DataFixture('Magento_InventoryConfigurableProductIndexer::Test/_files/source_items_configurable_multiple.php'),
+    ]
+    public function testTheParentStaysSalableWhenOnlyTheDefaultSourceRunsOut(): void
+    {
+        $before = $this->salabilityInNonDefaultStock();
+        self::assertSame(
+            1,
+            $before,
+            'PREMISE FAILED: the parent must be salable in the non-default stock to begin with, otherwise '
+            . 'this test cannot detect the regression it guards against.'
+        );
+
+        // The default source runs out. The eu sources backing stock 10 are untouched.
+        $this->setDefaultSourceOutOfStock();
+
+        self::assertSame(
+            1,
+            $this->salabilityInNonDefaultStock(),
+            'magento/inventory#3350 has been reintroduced: the parent lost its salability in a stock backed '
+            . 'by sources that still hold stock, because its default-stock legacy row was recomputed from '
+            . 'default-scope children and then vetoed the other stock.'
+        );
+    }
+
+    /**
+     * @return int
+     */
+    private function salabilityInNonDefaultStock(): int
+    {
+        $data = $this->getStockItemData->execute(self::PARENT_SKU, self::NON_DEFAULT_STOCK_ID);
+        self::assertNotNull(
+            $data,
+            sprintf('No index row for "%s" in stock %d.', self::PARENT_SKU, self::NON_DEFAULT_STOCK_ID)
+        );
+
+        return (int)$data[GetStockItemDataInterface::IS_SALABLE];
+    }
+
+    /**
+     * Put every child out of stock on the default source only.
+     *
+     * @return void
+     */
+    private function setDefaultSourceOutOfStock(): void
+    {
+        $objectManager = Bootstrap::getObjectManager();
+        $sourceItemFactory = $objectManager->get(SourceItemInterfaceFactory::class);
+
+        $sourceItems = [];
+        foreach (self::CHILD_SKUS as $sku) {
+            $sourceItem = $sourceItemFactory->create();
+            $sourceItem->setSourceCode('default');
+            $sourceItem->setSku($sku);
+            $sourceItem->setQuantity(0);
+            $sourceItem->setStatus(SourceItemInterface::STATUS_OUT_OF_STOCK);
+            $sourceItems[] = $sourceItem;
+        }
+
+        $objectManager->get(SourceItemsSaveInterface::class)->execute($sourceItems);
+    }
+}
diff --git a/InventoryCatalog/Test/Unit/Plugin/InventoryApi/UpdateCompositeProductStockStatusOnSourceItemsSaveTest.php b/InventoryCatalog/Test/Unit/Plugin/InventoryApi/UpdateCompositeProductStockStatusOnSourceItemsSaveTest.php
new file mode 100644
index 000000000000..80902e0a9527
--- /dev/null
+++ b/InventoryCatalog/Test/Unit/Plugin/InventoryApi/UpdateCompositeProductStockStatusOnSourceItemsSaveTest.php
@@ -0,0 +1,111 @@
+<?php
+/**
+ * Copyright 2026 Adobe
+ * All Rights Reserved.
+ */
+declare(strict_types=1);
+
+namespace Magento\InventoryCatalog\Test\Unit\Plugin\InventoryApi;
+
+use Magento\InventoryApi\Api\Data\SourceItemInterface;
+use Magento\InventoryApi\Api\SourceItemsSaveInterface;
+use Magento\InventoryCatalog\Plugin\InventoryApi\UpdateCompositeProductStockStatusOnSourceItemsSave;
+use Magento\InventoryCatalogApi\Model\CompositeProductStockStatusProcessorInterface;
+use Magento\InventoryCatalogApi\Model\IsSingleSourceModeInterface;
+use PHPUnit\Framework\MockObject\MockObject;
+use PHPUnit\Framework\TestCase;
+
+/**
+ * Documents the source-mode gate on the composite parent recompute.
+ *
+ * The recompute runs only in single source mode. In multi source mode it is skipped, because the parent's
+ * legacy stock item is a single default-stock value and recomputing it from default-scope children would
+ * let it veto stocks backed by other sources - see magento/inventory#3350.
+ */
+class UpdateCompositeProductStockStatusOnSourceItemsSaveTest extends TestCase
+{
+    /**
+     * @var IsSingleSourceModeInterface|MockObject
+     */
+    private $isSingleSourceMode;
+
+    /**
+     * @var CompositeProductStockStatusProcessorInterface|MockObject
+     */
+    private $processor;
+
+    /**
+     * @var UpdateCompositeProductStockStatusOnSourceItemsSave
+     */
+    private $plugin;
+
+    protected function setUp(): void
+    {
+        $this->isSingleSourceMode = $this->createMock(IsSingleSourceModeInterface::class);
+        $this->processor = $this->createMock(CompositeProductStockStatusProcessorInterface::class);
+        $this->plugin = new UpdateCompositeProductStockStatusOnSourceItemsSave(
+            $this->isSingleSourceMode,
+            $this->processor
+        );
+    }
+
+    /**
+     * @return void
+     */
+    public function testTheParentIsRecomputedInSingleSourceMode(): void
+    {
+        $this->isSingleSourceMode->method('execute')->willReturn(true);
+
+        $this->processor->expects($this->once())
+            ->method('execute')
+            ->with(['sku-a', 'sku-b']);
+
+        $this->plugin->afterExecute(
+            $this->createMock(SourceItemsSaveInterface::class),
+            null,
+            [$this->sourceItem('sku-a'), $this->sourceItem('sku-b')]
+        );
+    }
+
+    /**
+     * In multi source mode the recompute is deliberately skipped. See magento/inventory#3350: the parent's
+     * legacy row is default-stock data, and recomputing it here makes it veto other stocks.
+     *
+     * @return void
+     */
+    public function testTheParentIsNotRecomputedInMultiSourceMode(): void
+    {
+        $this->isSingleSourceMode->method('execute')->willReturn(false);
+
+        $this->processor->expects($this->never())->method('execute');
+
+        $this->plugin->afterExecute(
+            $this->createMock(SourceItemsSaveInterface::class),
+            null,
+            [$this->sourceItem('sku-a'), $this->sourceItem('sku-b')]
+        );
+    }
+
+    /**
+     * @return void
+     */
+    public function testAnEmptyBatchRecomputesNothing(): void
+    {
+        $this->isSingleSourceMode->method('execute')->willReturn(true);
+        $this->processor->expects($this->never())->method('execute');
+
+        $this->plugin->afterExecute($this->createMock(SourceItemsSaveInterface::class), null, []);
+    }
+
+    /**
+     * @param string $sku
+     * @return SourceItemInterface|MockObject
+     */
+    private function sourceItem(string $sku)
+    {
+        $sourceItem = $this->createMock(SourceItemInterface::class);
+        $sourceItem->method('getSku')->willReturn($sku);
+
+        return $sourceItem;
+    }
+}