diff --git a/Sources/JExtractSwiftLib/JavaPackageValidation.swift b/Sources/JExtractSwiftLib/JavaPackageValidation.swift
new file mode 100644
index 000000000..8bc811eef
--- /dev/null
+++ b/Sources/JExtractSwiftLib/JavaPackageValidation.swift
@@ -0,0 +1,51 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+/// Errors raised when `jextract` is configured without a usable Java package.
+package enum JavaPackageError: Error, Equatable, CustomStringConvertible {
+  /// `javaPackage` was `nil`, empty, whitespace-only, or had surrounding whitespace.
+  case missing
+  /// `javaPackage` contained an empty segment, e.g. `com..foo`, `.foo`, or `foo.`.
+  case emptySegment(String)
+
+  package var description: String {
+    switch self {
+    case .missing:
+      return
+        "Missing or empty Java package. jextract requires a non-empty '--java-package' (or javaPackage in swift-java.config). The unnamed/default package is not supported."
+    case .emptySegment(let javaPackage):
+      return "Invalid Java package '\(javaPackage)': must not contain empty segments."
+    }
+  }
+}
+
+/// Returns `javaPackage` unchanged when it is a non-empty, non-blank package name with no empty segments.
+///
+/// Surrounding whitespace is rejected rather than trimmed, so malformed configuration is not silently rewritten.
+package func validateJavaPackage(_ javaPackage: String?) throws -> String {
+  guard let javaPackage,
+    javaPackage == javaPackage.trimmingCharacters(in: .whitespacesAndNewlines),
+    !javaPackage.isEmpty
+  else {
+    throw JavaPackageError.missing
+  }
+
+  for component in javaPackage.split(separator: ".", omittingEmptySubsequences: false) {
+    if component.isEmpty {
+      throw JavaPackageError.emptySegment(javaPackage)
+    }
+  }
+
+  return javaPackage
+}
diff --git a/Sources/JExtractSwiftLib/Swift2Java.swift b/Sources/JExtractSwiftLib/Swift2Java.swift
index feff73c5f..6952f3b1e 100644
--- a/Sources/JExtractSwiftLib/Swift2Java.swift
+++ b/Sources/JExtractSwiftLib/Swift2Java.swift
@@ -35,15 +35,11 @@ public struct SwiftToJava {
       fatalError("Missing '--swift-module' name.")
     }
 
+    let javaPackage = try validateJavaPackage(config.javaPackage)
+
     let translator = makeSwiftJavaAnalyzer(config: config)
     let log = translator.log
 
-    if config.javaPackage == nil || config.javaPackage!.isEmpty {
-      translator.log.warning(
-        "Configured java package is '', consider specifying concrete package for generated sources."
-      )
-    }
-
     guard let inputSwift = config.inputSwiftDirectory else {
       fatalError("Missing '--swift-input' directory!")
     }
@@ -126,7 +122,7 @@ public struct SwiftToJava {
       let generator = FFMSwift2JavaGenerator(
         config: self.config,
         translator: translator,
-        javaPackage: config.javaPackage ?? "",
+        javaPackage: javaPackage,
         swiftOutputDirectory: outputSwiftDirectory,
         javaOutputDirectory: outputJavaDirectory
       )
@@ -137,7 +133,7 @@ public struct SwiftToJava {
       let generator = JNISwift2JavaGenerator(
         config: self.config,
         translator: translator,
-        javaPackage: config.javaPackage ?? "",
+        javaPackage: javaPackage,
         swiftOutputDirectory: outputSwiftDirectory,
         javaOutputDirectory: outputJavaDirectory,
         javaClassLookupTable: wrappedJavaClassesLookupTable,
diff --git a/Sources/SwiftJavaConfigurationShared/Configuration.swift b/Sources/SwiftJavaConfigurationShared/Configuration.swift
index 29c556afc..c45b00b2e 100644
--- a/Sources/SwiftJavaConfigurationShared/Configuration.swift
+++ b/Sources/SwiftJavaConfigurationShared/Configuration.swift
@@ -42,6 +42,8 @@ public struct Configuration: Codable {
 
   /// The Java package the generated Java code should be emitted into.
   ///
+  /// jextract requires a non-empty Java package name. The unnamed/default package is not supported.
+  ///
   /// Example:
   /// ```swift
   /// "com.example.mypackage"
diff --git a/Sources/SwiftJavaDocumentation/Documentation.docc/SwiftJavaConfigFile.md b/Sources/SwiftJavaDocumentation/Documentation.docc/SwiftJavaConfigFile.md
index 1ec58498f..bacce7ccf 100644
--- a/Sources/SwiftJavaDocumentation/Documentation.docc/SwiftJavaConfigFile.md
+++ b/Sources/SwiftJavaDocumentation/Documentation.docc/SwiftJavaConfigFile.md
@@ -54,6 +54,8 @@ The minimum log level at which log messages will be printed at by swift-java.
 
 The Java package the generated Java code should be emitted into.
 
+jextract requires a non-empty Java package name. The unnamed/default package is not supported.
+
 Example:
 ```swift
 "com.example.mypackage"
diff --git a/Tests/JExtractSwiftTests/JavaPackageValidationTests.swift b/Tests/JExtractSwiftTests/JavaPackageValidationTests.swift
new file mode 100644
index 000000000..c4a97ad3c
--- /dev/null
+++ b/Tests/JExtractSwiftTests/JavaPackageValidationTests.swift
@@ -0,0 +1,84 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift.org project authors
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+// See CONTRIBUTORS.txt for the list of Swift.org project authors
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftJavaConfigurationShared
+import Testing
+
+@testable import JExtractSwiftLib
+
+@Suite
+struct JavaPackageValidationTests {
+  @Test func rejectsNil() {
+    #expect(throws: JavaPackageError.missing) {
+      try validateJavaPackage(nil)
+    }
+  }
+
+  @Test func rejectsEmpty() {
+    #expect(throws: JavaPackageError.missing) {
+      try validateJavaPackage("")
+    }
+  }
+
+  @Test func rejectsWhitespaceOnly() {
+    #expect(throws: JavaPackageError.missing) {
+      try validateJavaPackage("   ")
+    }
+  }
+
+  @Test func rejectsSurroundingWhitespace() {
+    #expect(throws: JavaPackageError.missing) {
+      try validateJavaPackage(" com.example.foo ")
+    }
+  }
+
+  @Test func rejectsEmptySegment() {
+    #expect(throws: JavaPackageError.emptySegment("com..foo")) {
+      try validateJavaPackage("com..foo")
+    }
+  }
+
+  @Test func rejectsLeadingDot() {
+    #expect(throws: JavaPackageError.emptySegment(".foo")) {
+      try validateJavaPackage(".foo")
+    }
+  }
+
+  @Test func rejectsTrailingDot() {
+    #expect(throws: JavaPackageError.emptySegment("foo.")) {
+      try validateJavaPackage("foo.")
+    }
+  }
+
+  @Test func acceptsNonEmptyPackageUnchanged() throws {
+    #expect(try validateJavaPackage("com.example.foo") == "com.example.foo")
+  }
+
+  @Test func runThrowsOnNilJavaPackage() {
+    var config = Configuration()
+    config.swiftModule = "MySwift"
+    #expect(throws: JavaPackageError.missing) {
+      try SwiftToJava(config: config, dependencyConfigs: []).run()
+    }
+  }
+
+  @Test func runThrowsOnEmptyJavaPackage() {
+    var config = Configuration()
+    config.swiftModule = "MySwift"
+    config.javaPackage = ""
+    #expect(throws: JavaPackageError.missing) {
+      try SwiftToJava(config: config, dependencyConfigs: []).run()
+    }
+  }
+}