diff --git a/build_overrides/pigweed.gni b/build_overrides/pigweed.gni
index 96486b1..8985dc7 100644
--- a/build_overrides/pigweed.gni
+++ b/build_overrides/pigweed.gni
@@ -38,4 +38,6 @@
       get_path_info("//pw_spin_delay_stm32f429i_disc1", "abspath")
   dir_pw_spin_delay_stm32cubef4 =
       get_path_info("//pw_spin_delay_stm32cubef4", "abspath")
+  dir_pw_sys_io_stm32cubef4 =
+      get_path_info("//pw_sys_io_stm32cubef4", "abspath")
 }
diff --git a/pw_sys_io_stm32cubef4/BUILD.gn b/pw_sys_io_stm32cubef4/BUILD.gn
new file mode 100644
index 0000000..f783f21
--- /dev/null
+++ b/pw_sys_io_stm32cubef4/BUILD.gn
@@ -0,0 +1,57 @@
+# Copyright 2021 The Pigweed Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License"); you may not
+# use this file except in compliance with the License. You may obtain a copy of
+# the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+# License for the specific language governing permissions and limitations under
+# the License.
+
+import("//build_overrides/pigweed.gni")
+
+import("$dir_pw_build/target_types.gni")
+import("$dir_pw_docgen/docs.gni")
+
+# Defaults to USART1, but can be overridden.
+declare_args() {
+  # USART to use for sys_io
+  pw_sys_io_stm32cubef4_usart_num = "1"
+
+  # GPIO Port of the specified USART
+  pw_sys_io_stm32cubef4_gpio_port = "A"
+
+  # GPIO Pin for tx
+  pw_sys_io_stm32cubef4_gpio_tx_pin = "9"
+
+  # GPIO Pin for rx
+  pw_sys_io_stm32cubef4_gpio_rx_pin = "10"
+}
+
+config("default_config") {
+  cflags = [
+    "-DUSART_NUM=" + pw_sys_io_stm32cubef4_usart_num,
+    "-DUSART_GPIO_PORT_CHAR=" + pw_sys_io_stm32cubef4_gpio_port,
+    "-DUSART_TX_PIN_NUM=" + pw_sys_io_stm32cubef4_gpio_tx_pin,
+    "-DUSART_RX_PIN_NUM=" + pw_sys_io_stm32cubef4_gpio_rx_pin,
+  ]
+  include_dirs = [ "public" ]
+}
+
+pw_source_set("pw_sys_io_stm32cubef4") {
+  public_configs = [ ":default_config" ]
+  public = []
+  public_deps = [
+    "$dir_pw_status",
+    "//third_party/stm32cubef4:stm32f4xx_hal",
+  ]
+  deps = [
+    "$dir_pw_sys_io:default_putget_bytes",
+    "$dir_pw_sys_io:facade",
+  ]
+  sources = [ "sys_io.cc" ]
+}
diff --git a/pw_sys_io_stm32cubef4/public/pw_sys_io_stm32cubef4/init.h b/pw_sys_io_stm32cubef4/public/pw_sys_io_stm32cubef4/init.h
new file mode 100644
index 0000000..b23cb51
--- /dev/null
+++ b/pw_sys_io_stm32cubef4/public/pw_sys_io_stm32cubef4/init.h
@@ -0,0 +1,23 @@
+// Copyright 2021 The Pigweed Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+#pragma once
+
+#include "pw_preprocessor/util.h"
+
+PW_EXTERN_C_START
+
+// The actual implement of PreMainInit() in sys_io_BACKEND.
+void pw_sys_io_Init();
+
+PW_EXTERN_C_END
diff --git a/pw_sys_io_stm32cubef4/sys_io.cc b/pw_sys_io_stm32cubef4/sys_io.cc
new file mode 100644
index 0000000..4744a2b
--- /dev/null
+++ b/pw_sys_io_stm32cubef4/sys_io.cc
@@ -0,0 +1,99 @@
+// Copyright 2021 The Pigweed Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License"); you may not
+// use this file except in compliance with the License. You may obtain a copy of
+// the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+// WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+// License for the specific language governing permissions and limitations under
+// the License.
+
+#include "pw_sys_io/sys_io.h"
+
+#include <cinttypes>
+
+#include "pw_status/status.h"
+#include "stm32f4xx.h"
+
+#define _CAT(A, B, C) A##B##C
+#define CAT(A, B, C) _CAT(A, B, C)
+
+#define USART_INSTANCE CAT(USART, USART_NUM, )
+#define USART_GPIO_ALTERNATE_FUNC CAT(GPIO_AF7_USART, USART_NUM, )
+#define USART_GPIO_PORT CAT(GPIO, USART_GPIO_PORT_CHAR, )
+#define USART_GPIO_TX_PIN CAT(GPIO_PIN_, USART_TX_PIN_NUM, )
+#define USART_GPIO_RX_PIN CAT(GPIO_PIN_, USART_RX_PIN_NUM, )
+
+#define USART_GPIO_PORT_ENABLE \
+  CAT(__HAL_RCC_GPIO, USART_GPIO_PORT_CHAR, _CLK_ENABLE)
+#define USART_ENABLE CAT(__HAL_RCC_USART, USART_NUM, _CLK_ENABLE)
+
+static UART_HandleTypeDef uart;
+
+extern "C" void pw_sys_io_Init() {
+  GPIO_InitTypeDef GPIO_InitStruct = {};
+
+  USART_ENABLE();
+  USART_GPIO_PORT_ENABLE();
+
+  GPIO_InitStruct.Pin = USART_GPIO_TX_PIN | USART_GPIO_RX_PIN;
+  GPIO_InitStruct.Mode = GPIO_MODE_AF_PP;
+  GPIO_InitStruct.Pull = GPIO_NOPULL;
+  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
+  GPIO_InitStruct.Alternate = USART_GPIO_ALTERNATE_FUNC;
+  HAL_GPIO_Init(USART_GPIO_PORT, &GPIO_InitStruct);
+
+  uart.Instance = USART_INSTANCE;
+  uart.Init.BaudRate = 115200;
+  uart.Init.WordLength = UART_WORDLENGTH_8B;
+  uart.Init.StopBits = UART_STOPBITS_1;
+  uart.Init.Parity = UART_PARITY_NONE;
+  uart.Init.Mode = UART_MODE_TX_RX;
+  uart.Init.HwFlowCtl = UART_HWCONTROL_NONE;
+  uart.Init.OverSampling = UART_OVERSAMPLING_16;
+  HAL_UART_Init(&uart);
+}
+
+// This whole implementation is very inefficient because it uses the synchronous
+// polling UART API and only reads / writes 1 byte at a time.
+namespace pw::sys_io {
+Status ReadByte(std::byte* dest) {
+  if (HAL_UART_Receive(
+          &uart, reinterpret_cast<uint8_t*>(dest), 1, HAL_MAX_DELAY) !=
+      HAL_OK) {
+    return Status::ResourceExhausted();
+  }
+  return OkStatus();
+}
+
+Status TryReadByte(std::byte* dest) { return Status::Unimplemented(); }
+
+Status WriteByte(std::byte b) {
+  if (HAL_UART_Transmit(
+          &uart, reinterpret_cast<uint8_t*>(&b), 1, HAL_MAX_DELAY) != HAL_OK) {
+    return Status::ResourceExhausted();
+  }
+  return OkStatus();
+}
+
+// Writes a string using pw::sys_io, and add newline characters at the end.
+StatusWithSize WriteLine(const std::string_view& s) {
+  size_t chars_written = 0;
+  StatusWithSize result = WriteBytes(std::as_bytes(std::span(s)));
+  if (!result.ok()) {
+    return result;
+  }
+  chars_written += result.size();
+
+  // Write trailing newline.
+  result = WriteBytes(std::as_bytes(std::span("\r\n", 2)));
+  chars_written += result.size();
+
+  return StatusWithSize(OkStatus(), chars_written);
+}
+
+}  // namespace pw::sys_io