diff --git a/core/src/main/java/org/testcontainers/utility/TestcontainersConfiguration.java b/core/src/main/java/org/testcontainers/utility/TestcontainersConfiguration.java
index 5c80b82ba9c..a332daae0f9 100644
--- a/core/src/main/java/org/testcontainers/utility/TestcontainersConfiguration.java
+++ b/core/src/main/java/org/testcontainers/utility/TestcontainersConfiguration.java
@@ -22,6 +22,7 @@
 import java.io.OutputStream;
 import java.net.MalformedURLException;
 import java.net.URL;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Optional;
 import java.util.Properties;
@@ -231,7 +232,7 @@ private String getConfigurable(
         @Nullable final String defaultValue,
         Properties... propertiesSources
     ) {
-        String envVarName = propertyName.replaceAll("\\.", "_").toUpperCase();
+        String envVarName = propertyName.replaceAll("\\.", "_").toUpperCase(Locale.ROOT);
         if (!envVarName.startsWith("TESTCONTAINERS_") && !envVarName.startsWith("DOCKER_")) {
             envVarName = "TESTCONTAINERS_" + envVarName;
         }
diff --git a/core/src/test/java/org/testcontainers/utility/TestcontainersConfigurationTest.java b/core/src/test/java/org/testcontainers/utility/TestcontainersConfigurationTest.java
index 4e0520af5ba..489b0ffc72c 100644
--- a/core/src/test/java/org/testcontainers/utility/TestcontainersConfigurationTest.java
+++ b/core/src/test/java/org/testcontainers/utility/TestcontainersConfigurationTest.java
@@ -4,6 +4,7 @@
 import org.junit.jupiter.api.Test;
 
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Properties;
 import java.util.UUID;
@@ -225,6 +226,24 @@ void shouldTrimImageNames() {
             .isEqualTo("testcontainers/ryuk:0.3.2");
     }
 
+    @Test
+    void shouldReadEnvVarRegardlessOfDefaultLocale() {
+        // Property names such as "image.substitutor" contain the letter 'i', which
+        // upper-cases to a dotted capital 'İ' (U+0130) under the Turkish locale. The
+        // derived environment variable name must not depend on the JVM default locale,
+        // otherwise the lookup silently fails and the environment variable is ignored.
+        Locale previousDefault = Locale.getDefault();
+        try {
+            Locale.setDefault(new Locale("tr", "TR"));
+            environment.put("TESTCONTAINERS_IMAGE_SUBSTITUTOR", "com.example.MySubstitutor");
+            assertThat(newConfig().getImageSubstitutorClassName())
+                .as("environment variables are resolved independently of the default locale")
+                .isEqualTo("com.example.MySubstitutor");
+        } finally {
+            Locale.setDefault(previousDefault);
+        }
+    }
+
     private TestcontainersConfiguration newConfig() {
         return new TestcontainersConfiguration(userProperties, classpathProperties, environment);
     }