diff --git a/esphome/components/sx1509/__init__.py b/esphome/components/sx1509/__init__.py
index 7694b8f732..e4df7b443c 100644
--- a/esphome/components/sx1509/__init__.py
+++ b/esphome/components/sx1509/__init__.py
@@ -43,6 +43,10 @@ SX1509KeyTrigger = sx1509_ns.class_(
 
 
 def check_keys(config: ConfigType) -> ConfigType:
+    for ch in config.get(CONF_KEYS, ""):
+        if not ch.isascii():
+            # Each key is reported as one byte, so only ASCII characters can be key codes
+            raise cv.Invalid(f"Key code {ch!r} is not an ASCII character")
     if (
         CONF_KEYS in config
         and len(config[CONF_KEYS]) != config[CONF_KEY_ROWS] * config[CONF_KEY_COLUMNS]
@@ -98,8 +102,11 @@ async def to_code(config: ConfigType) -> None:
             cg.add(var.set_sleep_time(conf[CONF_SLEEP_TIME]))
             cg.add(var.set_scan_time(conf[CONF_SCAN_TIME]))
             cg.add(var.set_debounce_time(conf[CONF_DEBOUNCE_TIME]))
-        if keys := conf.get(CONF_KEYS):
-            cg.add(var.set_keys(keys))
+        if (keys := conf.get(CONF_KEYS)) is not None:
+            table = cg.shared_progmem_array(
+                "sx1509_keys", cg.uint8, list(keys.encode())
+            )
+            cg.add(var.set_keys(table))
         for tconf in conf.get(CONF_ON_KEY, []):
             trigger = cg.new_Pvariable(tconf[CONF_TRIGGER_ID])
             cg.add(var.register_key_trigger(trigger))
diff --git a/esphome/components/sx1509/sx1509.cpp b/esphome/components/sx1509/sx1509.cpp
index 2397049000..ee8606db96 100644
--- a/esphome/components/sx1509/sx1509.cpp
+++ b/esphome/components/sx1509/sx1509.cpp
@@ -48,7 +48,7 @@ void SX1509Component::loop() {
     uint16_t key_data = this->read_key_data();
     for (auto *binary_sensor : this->keypad_binary_sensors_)
       binary_sensor->process(key_data);
-    if (this->keys_.empty())
+    if (this->keys_ == nullptr)
       return;
     if (key_data == 0) {
       this->last_key_ = 0;
@@ -64,7 +64,9 @@ void SX1509Component::loop() {
         break;
     }
     col -= 8;
-    uint8_t key = this->keys_[row * this->cols_ + col];
+    if (row >= this->rows_ || col >= this->cols_)  // a partial read can leave only a row or a column bit
+      return;
+    uint8_t key = progmem_read_byte(&this->keys_[row * this->cols_ + col]);
     if (key == this->last_key_)
       return;
     this->last_key_ = key;
diff --git a/esphome/components/sx1509/sx1509.h b/esphome/components/sx1509/sx1509.h
index c7aed2cddd..ef6ee6ec5e 100644
--- a/esphome/components/sx1509/sx1509.h
+++ b/esphome/components/sx1509/sx1509.h
@@ -51,7 +51,8 @@ class SX1509Component final : public Component,
     this->cols_ = cols;
     this->has_keypad_ = true;
   };
-  void set_keys(std::string keys) { this->keys_ = std::move(keys); };  // NOLINT(performance-unnecessary-value-param)
+  /// `keys` is a codegen PROGMEM table with one key code per button.
+  void set_keys(const uint8_t *keys) { this->keys_ = keys; }
   void set_sleep_time(uint16_t sleep_time) { this->sleep_time_ = sleep_time; };
   void set_scan_time(uint8_t scan_time) { this->scan_time_ = scan_time; };
   void set_debounce_time(uint8_t debounce_time = 1) { this->debounce_time_ = debounce_time; };
@@ -76,7 +77,7 @@ class SX1509Component final : public Component,
   bool has_keypad_ = false;
   uint8_t rows_ = 0;
   uint8_t cols_ = 0;
-  std::string keys_;
+  const uint8_t *keys_{nullptr};
   uint16_t sleep_time_ = 128;
   uint8_t scan_time_ = 1;
   uint8_t debounce_time_ = 1;
diff --git a/tests/component_tests/sx1509/__init__.py b/tests/component_tests/sx1509/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/tests/component_tests/sx1509/config/keys.yaml b/tests/component_tests/sx1509/config/keys.yaml
new file mode 100644
index 0000000000..e5be4554c9
--- /dev/null
+++ b/tests/component_tests/sx1509/config/keys.yaml
@@ -0,0 +1,23 @@
+esphome:
+  name: test
+
+esp8266:
+  board: esp01_1m
+
+i2c:
+  sda: 4
+  scl: 5
+
+sx1509:
+  - id: sx_a
+    address: 0x3E
+    keypad:
+      key_rows: 2
+      key_columns: 2
+      keys: "abcd"
+  - id: sx_b
+    address: 0x3F
+    keypad:
+      key_rows: 2
+      key_columns: 2
+      keys: "abcd"
diff --git a/tests/component_tests/sx1509/test_keys.py b/tests/component_tests/sx1509/test_keys.py
new file mode 100644
index 0000000000..d7d41ae525
--- /dev/null
+++ b/tests/component_tests/sx1509/test_keys.py
@@ -0,0 +1,27 @@
+"""SX1509 keypad key codes live in a shared PROGMEM table instead of a std::string."""
+
+from collections.abc import Callable
+from pathlib import Path
+
+import pytest
+
+from esphome.components.sx1509 import check_keys
+import esphome.config_validation as cv
+
+
+def test_keys_use_shared_progmem_table(
+    generate_main: Callable[[str | Path], str],
+    component_config_path: Callable[[str], Path],
+) -> None:
+    main_cpp = generate_main(component_config_path("keys.yaml"))
+
+    assert (
+        "static constexpr uint8_t sx1509_keys[] PROGMEM = {97, 98, 99, 100};"
+        in main_cpp
+    )
+    assert main_cpp.count("->set_keys(sx1509_keys);") == 2
+
+
+def test_non_ascii_keys_are_rejected() -> None:
+    with pytest.raises(cv.Invalid, match="'é' is not an ASCII"):
+        check_keys({"keys": "1é34"})