diff --git a/esphome/components/modbus/modbus.cpp b/esphome/components/modbus/modbus.cpp
index 28e26e307e3..da5bfa5ef85 100644
--- a/esphome/components/modbus/modbus.cpp
+++ b/esphome/components/modbus/modbus.cpp
@@ -21,8 +21,14 @@ void Modbus::setup() {
                    // 3.5 characters * 11 bits per character * 1000ms/sec / (bits/sec) (Standard modbus frame delay)
                (uint16_t) (3.5 * 11 * 1000 / this->parent_->get_baud_rate()) + 1);
 
-  this->long_rx_buffer_delay_ms_ =
-      (this->parent_->get_rx_full_threshold() * 11 * 1000 / this->parent_->get_baud_rate()) + 1;
+  // When rx_full_threshold is configured (non-zero), the UART has a hardware FIFO with a
+  // meaningful threshold (e.g., ESP32 native UART), so we can calculate a precise delay.
+  // Otherwise (e.g., USB UART), use 50ms to handle data arriving in chunks.
+  static constexpr uint16_t DEFAULT_LONG_RX_BUFFER_DELAY_MS = 50;
+  size_t rx_threshold = this->parent_->get_rx_full_threshold();
+  this->long_rx_buffer_delay_ms_ = rx_threshold != uart::UARTComponent::RX_FULL_THRESHOLD_UNSET
+                                       ? (rx_threshold * 11 * 1000 / this->parent_->get_baud_rate()) + 1
+                                       : DEFAULT_LONG_RX_BUFFER_DELAY_MS;
 }
 
 void Modbus::loop() {
diff --git a/esphome/components/uart/uart_component.h b/esphome/components/uart/uart_component.h
index 078ce64b30f..3d2a848462e 100644
--- a/esphome/components/uart/uart_component.h
+++ b/esphome/components/uart/uart_component.h
@@ -187,7 +187,10 @@ class UARTComponent {
   InternalGPIOPin *rx_pin_{};
   InternalGPIOPin *flow_control_pin_{};
   size_t rx_buffer_size_{};
-  size_t rx_full_threshold_{1};
+  static constexpr size_t RX_FULL_THRESHOLD_UNSET = 0;
+  // ESP-IDF always sets this at codegen time via set_rx_full_threshold().
+  // Other platforms (USB UART, Arduino, etc.) leave it unset.
+  size_t rx_full_threshold_{RX_FULL_THRESHOLD_UNSET};
   size_t rx_timeout_{0};
   uint32_t baud_rate_{0};
   uint8_t stop_bits_{0};