diff --git a/.changelog/5705.fixed b/.changelog/5705.fixed
new file mode 100644
index 00000000000..6165ab0a5b8
--- /dev/null
+++ b/.changelog/5705.fixed
@@ -0,0 +1 @@
+`opentelemetry-exporter-prometheus`: warn and fall back to the default port instead of raising `ValueError` when `OTEL_EXPORTER_PROMETHEUS_PORT` holds a non-integer value in the auto-configured metric reader.
diff --git a/exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py b/exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py
index 0fef403b90a..a709e875542 100644
--- a/exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py
+++ b/exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py
@@ -113,6 +113,32 @@
 _TARGET_INFO_NAME = "target"
 _TARGET_INFO_DESCRIPTION = "Target metadata"
 
+_DEFAULT_PORT = 9464
+
+
+def _port_from_env() -> int:
+    """Return ``OTEL_EXPORTER_PROMETHEUS_PORT`` as an int, defaulting to
+    ``_DEFAULT_PORT`` when the variable is unset, empty, or not an integer.
+
+    An invalid value is logged and ignored instead of raising, mirroring the
+    OTLP/Zipkin exporters so a malformed environment variable cannot take
+    auto-instrumentation startup down.
+    """
+    raw = environ.get(OTEL_EXPORTER_PROMETHEUS_PORT)
+    if raw is None or not raw.strip():
+        return _DEFAULT_PORT
+    try:
+        return int(raw)
+    except ValueError:
+        _logger.warning(
+            "Invalid value %r for %s, using default port %s",
+            raw,
+            OTEL_EXPORTER_PROMETHEUS_PORT,
+            _DEFAULT_PORT,
+        )
+        return _DEFAULT_PORT
+
+
 _OTEL_SCOPE_NAME_LABEL = "otel_scope_name"
 _OTEL_SCOPE_VERSION_LABEL = "otel_scope_version"
 _OTEL_SCOPE_SCHEMA_URL_LABEL = "otel_scope_schema_url"
@@ -518,6 +544,6 @@ def __init__(self) -> None:
         # Default values are specified in
         # https://github.com/open-telemetry/opentelemetry-specification/blob/v1.24.0/specification/configuration/sdk-environment-variables.md#prometheus-exporter
         start_http_server(
-            port=int(environ.get(OTEL_EXPORTER_PROMETHEUS_PORT, "9464")),
+            port=_port_from_env(),
             addr=environ.get(OTEL_EXPORTER_PROMETHEUS_HOST, "localhost"),
         )
diff --git a/exporter/opentelemetry-exporter-prometheus/tests/test_entrypoints.py b/exporter/opentelemetry-exporter-prometheus/tests/test_entrypoints.py
index 9eb7e037e3b..af00a4dedb1 100644
--- a/exporter/opentelemetry-exporter-prometheus/tests/test_entrypoints.py
+++ b/exporter/opentelemetry-exporter-prometheus/tests/test_entrypoints.py
@@ -52,3 +52,21 @@ def test_starts_http_server_host_envvar(self, mock_start_http_server: Mock) -> N
     def test_starts_http_server_port_envvar(self, mock_start_http_server: Mock) -> None:
         _AutoPrometheusMetricReader()
         mock_start_http_server.assert_called_once_with(port=9999, addr=ANY)
+
+    @patch("opentelemetry.exporter.prometheus.start_http_server")
+    @patch.dict(os.environ, {OTEL_EXPORTER_PROMETHEUS_PORT: "9464abc"}, clear=False)
+    def test_starts_http_server_invalid_port_envvar_falls_back(self, mock_start_http_server: Mock) -> None:
+        with self.assertLogs("opentelemetry.exporter.prometheus", level="WARNING") as cm:
+            _AutoPrometheusMetricReader()
+
+        mock_start_http_server.assert_called_once_with(port=9464, addr=ANY)
+        self.assertTrue(
+            any("Invalid value" in message for message in cm.output),
+            cm.output,
+        )
+
+    @patch("opentelemetry.exporter.prometheus.start_http_server")
+    @patch.dict(os.environ, {OTEL_EXPORTER_PROMETHEUS_PORT: ""}, clear=False)
+    def test_starts_http_server_empty_port_envvar_falls_back(self, mock_start_http_server: Mock) -> None:
+        _AutoPrometheusMetricReader()
+        mock_start_http_server.assert_called_once_with(port=9464, addr=ANY)