diff --git a/src/AutoMapper/Configuration/MapperConfigurationExpression.cs b/src/AutoMapper/Configuration/MapperConfigurationExpression.cs
index 6bfebeadfe..c71ba333fb 100644
--- a/src/AutoMapper/Configuration/MapperConfigurationExpression.cs
+++ b/src/AutoMapper/Configuration/MapperConfigurationExpression.cs
@@ -127,7 +127,13 @@ void IGlobalConfigurationExpression.Validator(Validator validator) =>
     /// </summary>
     int IGlobalConfigurationExpression.RecursiveQueriesMaxDepth { get; set; }
 
-    public string LicenseKey { get; set; }
+    private string _licenseKey;
+
+    public string LicenseKey
+    {
+        get => _licenseKey ?? Environment.GetEnvironmentVariable("AUTOMAPPER_LICENSE_KEY");
+        set => _licenseKey = value;
+    }
 
     public ServiceLifetime ServiceLifetime { get; set; } = ServiceLifetime.Transient;
 
diff --git a/src/UnitTests/Licensing/LicenseKeyEnvironmentVariableTests.cs b/src/UnitTests/Licensing/LicenseKeyEnvironmentVariableTests.cs
new file mode 100644
index 0000000000..27f135cc09
--- /dev/null
+++ b/src/UnitTests/Licensing/LicenseKeyEnvironmentVariableTests.cs
@@ -0,0 +1,208 @@
+using Microsoft.Extensions.DependencyInjection;
+
+namespace AutoMapper.UnitTests.Licensing;
+
+public class LicenseKeyEnvironmentVariableTests
+{
+    private const string EnvVarName = "AUTOMAPPER_LICENSE_KEY";
+
+    #region Environment Variable Auto-Detection Tests
+
+    [Fact]
+    public void LicenseKey_ReadsFromEnvironmentVariable_WhenNotExplicitlySet()
+    {
+        const string expectedKey = "test-license-key-12345";
+        Environment.SetEnvironmentVariable(EnvVarName, expectedKey);
+
+        try
+        {
+            var config = new MapperConfigurationExpression();
+
+            config.LicenseKey.ShouldBe(expectedKey);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(EnvVarName, null);
+        }
+    }
+
+    [Fact]
+    public void LicenseKey_ReturnsNull_WhenEnvironmentVariableNotSet()
+    {
+        Environment.SetEnvironmentVariable(EnvVarName, null);
+
+        var config = new MapperConfigurationExpression();
+
+        config.LicenseKey.ShouldBeNull();
+    }
+
+    #endregion
+
+    #region Backward Compatibility - Old Way Tests
+
+    [Fact]
+    public void LicenseKey_SupportsOldWay_DirectAssignment()
+    {
+        const string licenseKey = "old-way-explicit-key";
+        var config = new MapperConfigurationExpression();
+
+        config.LicenseKey = licenseKey;
+
+        config.LicenseKey.ShouldBe(licenseKey);
+    }
+
+    [Fact]
+    public void LicenseKey_PrioritizesExplicitValue_OverEnvironmentVariable()
+    {
+        const string envKey = "env-license-key";
+        const string explicitKey = "explicit-license-key";
+        Environment.SetEnvironmentVariable(EnvVarName, envKey);
+
+        try
+        {
+            var config = new MapperConfigurationExpression();
+
+            config.LicenseKey = explicitKey;
+
+            config.LicenseKey.ShouldBe(explicitKey);
+            config.LicenseKey.ShouldNotBe(envKey);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(EnvVarName, null);
+        }
+    }
+
+    [Fact]
+    public void LicenseKey_OldWayOverridesEnvironmentVariable_InConfigAction()
+    {
+        const string envKey = "env-license-key";
+        const string explicitKey = "explicit-override-key";
+        Environment.SetEnvironmentVariable(EnvVarName, envKey);
+
+        try
+        {
+            var config = new MapperConfigurationExpression();
+
+            config.LicenseKey = explicitKey;
+
+            config.LicenseKey.ShouldBe(explicitKey);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(EnvVarName, null);
+        }
+    }
+
+    #endregion
+
+    #region Integration Tests - Old Way with DI
+
+    [Fact]
+    public void AddAutoMapper_SupportsOldWay_ExplicitLicenseKey()
+    {
+        const string licenseKey = "old-way-integration-key";
+        MapperConfigurationExpression capturedCfg = null;
+
+        var services = new ServiceCollection();
+        services.AddLogging();
+        services.AddAutoMapper(cfg =>
+        {
+            cfg.LicenseKey = licenseKey;
+            cfg.CreateMap<TestSource, TestDestination>();
+            capturedCfg = (MapperConfigurationExpression)cfg;
+        });
+
+        var serviceProvider = services.BuildServiceProvider();
+        var mapper = serviceProvider.GetRequiredService<IMapper>();
+
+        var result = mapper.Map<TestDestination>(new TestSource { Name = "Test" });
+
+        result.ShouldNotBeNull();
+        result.Name.ShouldBe("Test");
+        capturedCfg.LicenseKey.ShouldBe(licenseKey);
+    }
+
+    [Fact]
+    public void AddAutoMapper_UsesEnvironmentVariable_WhenNoExplicitKeySet()
+    {
+        const string licenseKey = "env-integration-key";
+        Environment.SetEnvironmentVariable(EnvVarName, licenseKey);
+        MapperConfigurationExpression capturedCfg = null;
+
+        try
+        {
+            var services = new ServiceCollection();
+            services.AddLogging();
+            services.AddAutoMapper(cfg =>
+            {
+                cfg.CreateMap<TestSource, TestDestination>();
+                capturedCfg = (MapperConfigurationExpression)cfg;
+            });
+
+            var serviceProvider = services.BuildServiceProvider();
+            var mapper = serviceProvider.GetRequiredService<IMapper>();
+
+            var result = mapper.Map<TestDestination>(new TestSource { Name = "Test" });
+
+            result.ShouldNotBeNull();
+            result.Name.ShouldBe("Test");
+            capturedCfg.LicenseKey.ShouldBe(licenseKey);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(EnvVarName, null);
+        }
+    }
+
+    [Fact]
+    public void AddAutoMapper_OldWayTakesPrecedence_OverEnvironmentVariable()
+    {
+        const string envKey = "env-license-key";
+        const string explicitKey = "explicit-license-key-from-old-way";
+        Environment.SetEnvironmentVariable(EnvVarName, envKey);
+        MapperConfigurationExpression capturedCfg = null;
+
+        try
+        {
+            var services = new ServiceCollection();
+            services.AddLogging();
+            services.AddAutoMapper(cfg =>
+            {
+                cfg.LicenseKey = explicitKey;
+                cfg.CreateMap<TestSource, TestDestination>();
+                capturedCfg = (MapperConfigurationExpression)cfg;
+            });
+
+            var serviceProvider = services.BuildServiceProvider();
+            var mapper = serviceProvider.GetRequiredService<IMapper>();
+
+            var result = mapper.Map<TestDestination>(new TestSource { Name = "Test" });
+
+            result.ShouldNotBeNull();
+            result.Name.ShouldBe("Test");
+            capturedCfg.LicenseKey.ShouldBe(explicitKey);
+            capturedCfg.LicenseKey.ShouldNotBe(envKey);
+        }
+        finally
+        {
+            Environment.SetEnvironmentVariable(EnvVarName, null);
+        }
+    }
+
+    #endregion
+
+    #region Test Helper Classes
+
+    private class TestSource
+    {
+        public string Name { get; set; }
+    }
+
+    private class TestDestination
+    {
+        public string Name { get; set; }
+    }
+
+    #endregion
+}
\ No newline at end of file