diff --git a/esphome/components/ota_base/ota_backend_arduino_esp32.cpp b/esphome/components/ota_base/ota_backend_arduino_esp32.cpp
index 34ba3ae6ff..b89c61472a 100644
--- a/esphome/components/ota_base/ota_backend_arduino_esp32.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_esp32.cpp
@@ -15,6 +15,11 @@ static const char *const TAG = "ota.arduino_esp32";
 std::unique_ptr<OTABackend> make_ota_backend() { return make_unique<ArduinoESP32OTABackend>(); }
 
 OTAResponseTypes ArduinoESP32OTABackend::begin(size_t image_size) {
+  // Handle UPDATE_SIZE_UNKNOWN (0) which is used by web server OTA
+  // where the exact firmware size is unknown due to multipart encoding
+  if (image_size == 0) {
+    image_size = UPDATE_SIZE_UNKNOWN;
+  }
   bool ret = Update.begin(image_size, U_FLASH);
   if (ret) {
     return OTA_RESPONSE_OK;
diff --git a/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp b/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp
index 12d4b677a3..052a9faed9 100644
--- a/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_libretiny.cpp
@@ -15,6 +15,11 @@ static const char *const TAG = "ota.arduino_libretiny";
 std::unique_ptr<OTABackend> make_ota_backend() { return make_unique<ArduinoLibreTinyOTABackend>(); }
 
 OTAResponseTypes ArduinoLibreTinyOTABackend::begin(size_t image_size) {
+  // Handle UPDATE_SIZE_UNKNOWN (0) which is used by web server OTA
+  // where the exact firmware size is unknown due to multipart encoding
+  if (image_size == 0) {
+    image_size = UPDATE_SIZE_UNKNOWN;
+  }
   bool ret = Update.begin(image_size, U_FLASH);
   if (ret) {
     return OTA_RESPONSE_OK;
diff --git a/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp b/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp
index 7276381919..bcb87f3547 100644
--- a/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp
+++ b/esphome/components/ota_base/ota_backend_arduino_rp2040.cpp
@@ -17,6 +17,11 @@ static const char *const TAG = "ota.arduino_rp2040";
 std::unique_ptr<OTABackend> make_ota_backend() { return make_unique<ArduinoRP2040OTABackend>(); }
 
 OTAResponseTypes ArduinoRP2040OTABackend::begin(size_t image_size) {
+  // Handle UPDATE_SIZE_UNKNOWN (0) which is used by web server OTA
+  // where the exact firmware size is unknown due to multipart encoding
+  if (image_size == 0) {
+    image_size = UPDATE_SIZE_UNKNOWN;
+  }
   bool ret = Update.begin(image_size, U_FLASH);
   if (ret) {
     rp2040::preferences_prevent_write(true);