diff --git a/api/v1/invoice_items/create.php b/api/v1/invoice_items/create.php
new file mode 100644
index 000000000..f8158fa36
--- /dev/null
+++ b/api/v1/invoice_items/create.php
@@ -0,0 +1,204 @@
+<?php
+
+/*
+ * API - Invoice Items - Create
+ * POST /api/v1/invoice_items/create.php
+ *
+ * Adds a line item to an existing invoice.
+ *
+ * Parameters (POST, JSON body):
+ *   api_key          required - Your API key
+ *   client_id        required - Must match the invoice's client (restricted
+ *                               keys only; unrestricted/admin keys may omit)
+ *   invoice_id       required - Invoice to add the item to
+ *   name             required - Item name
+ *   description      optional - Item description
+ *   qty              required - Quantity
+ *   price            required - Unit price
+ *   tax_id           optional - Tax ID (default 0)
+ *   item_order       optional - Display order (default 0)
+ *   product_id       optional - Product ID for inventory tracking
+ *
+ * Security:
+ *   - The parent invoice is loaded through apiClientScopeSql(), so a restricted
+ *     key cannot modify another client's invoice.
+ *   - The supplied client_id must match the invoice's client.
+ *   - Inventory is only adjusted for tangible products.
+ *   - Invoice totals are automatically recalculated.
+ */
+
+require_once '../validate_api_key.php';
+
+require_once '../require_post_method.php';
+
+// Parse input
+$invoice_id = intval($_POST['invoice_id'] ?? 0);
+$name = escapeSql(substr($_POST['name'], 0, 200) ?? '');
+$description = escapeSql($_POST['description'] ?? '');
+$qty = floatval($_POST['qty'] ?? 0);
+$price = floatval($_POST['price'] ?? 0);
+$tax_id = intval($_POST['tax_id'] ?? 0);
+$item_order = intval($_POST['item_order'] ?? 0);
+$product_id = intval($_POST['product_id'] ?? 0);
+
+$insert_id = false;
+
+if (
+    !empty($invoice_id)
+    && !empty($name)
+    && $qty > 0
+) {
+
+    // Load invoice, scoped to API key permissions
+    $invoice_sql = mysqli_query(
+        $mysqli,
+        "SELECT *
+         FROM invoices
+         WHERE invoice_id = $invoice_id
+         AND invoice_status != 'Paid'
+           AND 1=1 " . apiClientScopeSql('invoice_client_id') . "
+         LIMIT 1"
+    );
+
+    $invoice_row = $invoice_sql ? mysqli_fetch_assoc($invoice_sql) : null;
+
+    // Ensure supplied client matches invoice client
+    if ($invoice_row && $client_id != 0 && intval($invoice_row['invoice_client_id']) !== $client_id) {
+        $invoice_row = null;
+    }
+
+    if ($invoice_row) {
+
+        $client_id = intval($invoice_row['invoice_client_id']);
+        $invoice_prefix = escapeSql($invoice_row['invoice_prefix']);
+        $invoice_number = intval($invoice_row['invoice_number']);
+        $invoice_discount = floatval($invoice_row['invoice_discount_amount']);
+
+        $subtotal = $price * $qty;
+
+        // Product inventory
+        if ($product_id) {
+
+            $product_type = escapeSql(getFieldById('products', $product_id, 'product_type'));
+
+            if ($product_type === 'product') {
+
+                $stock_sql = mysqli_query(
+                    $mysqli,
+                    "SELECT COALESCE(SUM(stock_qty),0) AS available_stock
+                     FROM product_stock
+                     WHERE stock_product_id = $product_id"
+                );
+
+                $stock_row = mysqli_fetch_assoc($stock_sql);
+                $available_stock = floatval($stock_row['available_stock']);
+
+                if ($available_stock >= $qty) {
+
+                    mysqli_query(
+                        $mysqli,
+                        "INSERT INTO product_stock
+                         SET stock_qty = -$qty,
+                             stock_note = 'QTY $qty - Invoice $invoice_id',
+                             stock_product_id = $product_id"
+                    );
+
+                } else {
+
+                    logAudit(
+                        "API",
+                        "Failure",
+                        "Failed adding item $name to invoice $invoice_prefix$invoice_number via API ($api_key_name) due to insufficient stock",
+                        $client_id
+                    );
+
+                    require_once '../create_output.php';
+                    exit;
+
+                }
+
+            }
+
+        }
+
+        // Tax
+        if ($tax_id > 0) {
+
+            $tax_sql = mysqli_query($mysqli, "SELECT tax_percent FROM taxes WHERE tax_id = $tax_id");
+            $tax_row = mysqli_fetch_assoc($tax_sql);
+
+            $tax_percent = floatval($tax_row['tax_percent']);
+            $tax_amount = $subtotal * $tax_percent / 100;
+
+        } else {
+
+            $tax_amount = 0;
+
+        }
+
+        $total = $subtotal + $tax_amount;
+
+        $insert_sql = mysqli_query(
+            $mysqli,
+            "INSERT INTO invoice_items SET
+                item_name = '$name',
+                item_description = '$description',
+                item_quantity = $qty,
+                item_price = $price,
+                item_subtotal = $subtotal,
+                item_tax = $tax_amount,
+                item_total = $total,
+                item_order = $item_order,
+                item_tax_id = $tax_id,
+                item_product_id = $product_id,
+                item_invoice_id = $invoice_id"
+        );
+
+        if ($insert_sql) {
+
+            $insert_id = mysqli_insert_id($mysqli);
+
+            // Recalculate invoice total
+            $items_sql = mysqli_query(
+                $mysqli,
+                "SELECT SUM(item_total) AS invoice_total
+                 FROM invoice_items
+                 WHERE item_invoice_id = $invoice_id"
+            );
+
+            $items_row = mysqli_fetch_assoc($items_sql);
+            $invoice_total = floatval($items_row['invoice_total']);
+
+            $new_invoice_amount = $invoice_total - $invoice_discount;
+
+            mysqli_query(
+                $mysqli,
+                "UPDATE invoices
+                 SET invoice_amount = $new_invoice_amount
+                 WHERE invoice_id = $invoice_id
+                 LIMIT 1"
+            );
+
+            logAudit(
+                "Invoice",
+                "Edit",
+                "Added item $name to invoice $invoice_prefix$invoice_number via API ($api_key_name)",
+                $client_id,
+                $invoice_id
+            );
+
+            logAudit(
+                "API",
+                "Success",
+                "Added item $name to invoice $invoice_prefix$invoice_number via API ($api_key_name)",
+                $client_id
+            );
+
+        }
+
+    }
+
+}
+
+// Output
+require_once '../create_output.php';
\ No newline at end of file