diff --git a/CHANGELOG.md b/CHANGELOG.md
index dfc9179da3..352f5c89c9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Permit to override default HTTP OTLP exporters headers
+  ([#4634](https://github.com/open-telemetry/opentelemetry-python/pull/4634))
+
 ## Version 1.36.0/0.57b0 (2025-07-29)
 
 - Add missing Prometheus exporter documentation
diff --git a/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/_log_exporter/__init__.py b/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/_log_exporter/__init__.py
index 765bc5c7f5..ee29151b72 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/_log_exporter/__init__.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/_log_exporter/__init__.py
@@ -118,8 +118,9 @@ def __init__(
         )
         self._compression = compression or _compression_from_env()
         self._session = session or requests.Session()
-        self._session.headers.update(self._headers)
         self._session.headers.update(_OTLP_HTTP_HEADERS)
+        # let users override our defaults
+        self._session.headers.update(self._headers)
         if self._compression is not Compression.NoCompression:
             self._session.headers.update(
                 {"Content-Encoding": self._compression.value}
diff --git a/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/metric_exporter/__init__.py b/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/metric_exporter/__init__.py
index 3b7079f7fc..bcf4c29739 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/metric_exporter/__init__.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/metric_exporter/__init__.py
@@ -160,8 +160,9 @@ def __init__(
         )
         self._compression = compression or _compression_from_env()
         self._session = session or requests.Session()
-        self._session.headers.update(self._headers)
         self._session.headers.update(_OTLP_HTTP_HEADERS)
+        # let users override our defaults
+        self._session.headers.update(self._headers)
         if self._compression is not Compression.NoCompression:
             self._session.headers.update(
                 {"Content-Encoding": self._compression.value}
diff --git a/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/trace_exporter/__init__.py b/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/trace_exporter/__init__.py
index 8ea73d4c0f..1ed0400048 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/trace_exporter/__init__.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/trace_exporter/__init__.py
@@ -116,8 +116,9 @@ def __init__(
         )
         self._compression = compression or _compression_from_env()
         self._session = session or requests.Session()
-        self._session.headers.update(self._headers)
         self._session.headers.update(_OTLP_HTTP_HEADERS)
+        # let users override our defaults
+        self._session.headers.update(self._headers)
         if self._compression is not Compression.NoCompression:
             self._session.headers.update(
                 {"Content-Encoding": self._compression.value}
diff --git a/exporter/opentelemetry-exporter-otlp-proto-http/tests/metrics/test_otlp_metrics_exporter.py b/exporter/opentelemetry-exporter-otlp-proto-http/tests/metrics/test_otlp_metrics_exporter.py
index 815761397e..2453dc2dab 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-http/tests/metrics/test_otlp_metrics_exporter.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-http/tests/metrics/test_otlp_metrics_exporter.py
@@ -81,7 +81,7 @@
 OS_ENV_CERTIFICATE = "os/env/base.crt"
 OS_ENV_CLIENT_CERTIFICATE = "os/env/client-cert.pem"
 OS_ENV_CLIENT_KEY = "os/env/client-key.pem"
-OS_ENV_HEADERS = "envHeader1=val1,envHeader2=val2"
+OS_ENV_HEADERS = "envHeader1=val1,envHeader2=val2,User-agent=Overridden"
 OS_ENV_TIMEOUT = "30"
 
 
@@ -151,7 +151,7 @@ def test_constructor_default(self):
             OTEL_EXPORTER_OTLP_METRICS_CLIENT_KEY: "metrics/client-key.pem",
             OTEL_EXPORTER_OTLP_METRICS_COMPRESSION: Compression.Deflate.value,
             OTEL_EXPORTER_OTLP_METRICS_ENDPOINT: "https://metrics.endpoint.env",
-            OTEL_EXPORTER_OTLP_METRICS_HEADERS: "metricsEnv1=val1,metricsEnv2=val2,metricEnv3===val3==",
+            OTEL_EXPORTER_OTLP_METRICS_HEADERS: "metricsEnv1=val1,metricsEnv2=val2,metricEnv3===val3==,User-agent=metrics-user-agent",
             OTEL_EXPORTER_OTLP_METRICS_TIMEOUT: "40",
         },
     )
@@ -172,9 +172,18 @@ def test_exporter_metrics_env_take_priority(self):
                 "metricsenv1": "val1",
                 "metricsenv2": "val2",
                 "metricenv3": "==val3==",
+                "user-agent": "metrics-user-agent",
             },
         )
         self.assertIsInstance(exporter._session, Session)
+        self.assertEqual(
+            exporter._session.headers.get("User-Agent"),
+            "metrics-user-agent",
+        )
+        self.assertEqual(
+            exporter._session.headers.get("Content-Type"),
+            "application/x-protobuf",
+        )
 
     @patch.dict(
         "os.environ",
@@ -237,7 +246,12 @@ def test_exporter_env(self):
         self.assertEqual(exporter._timeout, int(OS_ENV_TIMEOUT))
         self.assertIs(exporter._compression, Compression.Gzip)
         self.assertEqual(
-            exporter._headers, {"envheader1": "val1", "envheader2": "val2"}
+            exporter._headers,
+            {
+                "envheader1": "val1",
+                "envheader2": "val2",
+                "user-agent": "Overridden",
+            },
         )
 
     @patch.dict(
diff --git a/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_log_exporter.py b/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_log_exporter.py
index 19183029ed..269b1143a8 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_log_exporter.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_log_exporter.py
@@ -71,7 +71,7 @@
 ENV_CERTIFICATE = "/etc/base.crt"
 ENV_CLIENT_CERTIFICATE = "/etc/client-cert.pem"
 ENV_CLIENT_KEY = "/etc/client-key.pem"
-ENV_HEADERS = "envHeader1=val1,envHeader2=val2"
+ENV_HEADERS = "envHeader1=val1,envHeader2=val2,User-agent=Overridden"
 ENV_TIMEOUT = "30"
 
 
@@ -114,7 +114,7 @@ def test_constructor_default(self):
             OTEL_EXPORTER_OTLP_LOGS_CLIENT_KEY: "logs/client-key.pem",
             OTEL_EXPORTER_OTLP_LOGS_COMPRESSION: Compression.Deflate.value,
             OTEL_EXPORTER_OTLP_LOGS_ENDPOINT: "https://logs.endpoint.env",
-            OTEL_EXPORTER_OTLP_LOGS_HEADERS: "logsEnv1=val1,logsEnv2=val2,logsEnv3===val3==",
+            OTEL_EXPORTER_OTLP_LOGS_HEADERS: "logsEnv1=val1,logsEnv2=val2,logsEnv3===val3==,User-agent=LogsUserAgent",
             OTEL_EXPORTER_OTLP_LOGS_TIMEOUT: "40",
         },
     )
@@ -135,9 +135,18 @@ def test_exporter_metrics_env_take_priority(self):
                 "logsenv1": "val1",
                 "logsenv2": "val2",
                 "logsenv3": "==val3==",
+                "user-agent": "LogsUserAgent",
             },
         )
         self.assertIsInstance(exporter._session, requests.Session)
+        self.assertEqual(
+            exporter._session.headers.get("User-Agent"),
+            "LogsUserAgent",
+        )
+        self.assertEqual(
+            exporter._session.headers.get("Content-Type"),
+            "application/x-protobuf",
+        )
 
     @patch.dict(
         "os.environ",
@@ -202,7 +211,12 @@ def test_exporter_env(self):
         self.assertEqual(exporter._timeout, int(ENV_TIMEOUT))
         self.assertIs(exporter._compression, Compression.Gzip)
         self.assertEqual(
-            exporter._headers, {"envheader1": "val1", "envheader2": "val2"}
+            exporter._headers,
+            {
+                "envheader1": "val1",
+                "envheader2": "val2",
+                "user-agent": "Overridden",
+            },
         )
         self.assertIsInstance(exporter._session, requests.Session)
 
diff --git a/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_span_exporter.py b/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_span_exporter.py
index 224227a7f5..f01a9efc91 100644
--- a/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_span_exporter.py
+++ b/exporter/opentelemetry-exporter-otlp-proto-http/tests/test_proto_span_exporter.py
@@ -54,7 +54,7 @@
 OS_ENV_CERTIFICATE = "os/env/base.crt"
 OS_ENV_CLIENT_CERTIFICATE = "os/env/client-cert.pem"
 OS_ENV_CLIENT_KEY = "os/env/client-key.pem"
-OS_ENV_HEADERS = "envHeader1=val1,envHeader2=val2"
+OS_ENV_HEADERS = "envHeader1=val1,envHeader2=val2,User-agent=Overridden"
 OS_ENV_TIMEOUT = "30"
 BASIC_SPAN = _Span(
     "abc",
@@ -108,7 +108,7 @@ def test_constructor_default(self):
             OTEL_EXPORTER_OTLP_TRACES_CLIENT_KEY: "traces/client-key.pem",
             OTEL_EXPORTER_OTLP_TRACES_COMPRESSION: Compression.Deflate.value,
             OTEL_EXPORTER_OTLP_TRACES_ENDPOINT: "https://traces.endpoint.env",
-            OTEL_EXPORTER_OTLP_TRACES_HEADERS: "tracesEnv1=val1,tracesEnv2=val2,traceEnv3===val3==",
+            OTEL_EXPORTER_OTLP_TRACES_HEADERS: "tracesEnv1=val1,tracesEnv2=val2,traceEnv3===val3==,User-agent=TraceUserAgent",
             OTEL_EXPORTER_OTLP_TRACES_TIMEOUT: "40",
         },
     )
@@ -129,9 +129,18 @@ def test_exporter_traces_env_take_priority(self):
                 "tracesenv1": "val1",
                 "tracesenv2": "val2",
                 "traceenv3": "==val3==",
+                "user-agent": "TraceUserAgent",
             },
         )
         self.assertIsInstance(exporter._session, requests.Session)
+        self.assertEqual(
+            exporter._session.headers.get("Content-Type"),
+            "application/x-protobuf",
+        )
+        self.assertEqual(
+            exporter._session.headers.get("User-Agent"),
+            "TraceUserAgent",
+        )
 
     @patch.dict(
         "os.environ",
@@ -194,7 +203,12 @@ def test_exporter_env(self):
         self.assertEqual(exporter._timeout, int(OS_ENV_TIMEOUT))
         self.assertIs(exporter._compression, Compression.Gzip)
         self.assertEqual(
-            exporter._headers, {"envheader1": "val1", "envheader2": "val2"}
+            exporter._headers,
+            {
+                "envheader1": "val1",
+                "envheader2": "val2",
+                "user-agent": "Overridden",
+            },
         )
 
     @patch.dict(