POBox Plus APK is an advanced Japanese input keyboard (IME) developed by Sony Mobile Communications for Xperia smartphones. The name “POBox” stands for "Predictive Operation Box", which highlights its main feature — powerful predictive text input designed for the Japanese language.
This keyboard is widely appreciated for its smooth typing experience, high-accuracy text prediction, and intelligent conversion between Hiragana, Katakana, and Kanji characters. It provides a native Japanese keyboard interface that feels natural and efficient, especially for users accustomed to Sony devices.
Over time, POBox Plus has evolved from older POBox versions, offering improved input accuracy, smarter contextual prediction, and modern visual themes that blend well with Sony’s Xperia user interface. Although it was originally built into Xperia devices, the APK version allows users to manually install it on compatible Android phones.

What is POBox Plus APK?

POBox Plus is a Japanese-language input (IME) keyboard app developed by Sony (or originally by Sony/Sony Mobile) for its Xperia line of smartphones.
It offers advanced Japanese text-input features tailored for Japanese users — including prediction, conversion, and other input convenience features.
In recent years, it’s not always pre-installed on newer Xperia devices, and some users extract the APK to install it manually on supported devices. The file on sites like APKMirror shows versions like 3.9.A.0.1, etc.

Features of POBox Plus APK:

Smart Prediction and Auto-Completion:

• POBox Plus learns from your typing patterns and frequently used phrases, improving prediction and reducing typing effort.

Advanced Japanese Conversion:

• It accurately converts Hiragana into Kanji and Katakana, ensuring that typed phrases match their intended meaning and grammatical structure.

Gesture and Swipe Input:

• Modern builds of POBox Plus include gesture-based typing, allowing you to input text with quick finger movements.

Custom Themes and Layouts:

• Offers multiple keyboard layouts — including 12-key Japanese (flick input), QWERTY layout, and half-width/ full-width modes.
  You can also customize the appearance to fit your Xperia theme.

Emoji and Symbol Support:

• Comes with rich emoji, kaomoji (Japanese text faces like ^_^), and symbol libraries for expressive communication.

Multilingual Input:

• While primarily Japanese-focused, some versions allow quick switching between English and Japanese for bilingual users.

Integration with Xperia UI:

• Works seamlessly on Xperia devices, integrating with system settings and text fields for a unified typing experience.

Offline Operation:

• Unlike cloud-based keyboards, POBox Plus performs predictions and conversions offline, ensuring privacy and consistent performance even without internet access.

How to Download and Install POBox Plus APK

If you want to install POBox Plus manually (for example, on a supported Xperia device), here are the broad steps. Note: Compatibility and success are not guaranteed, especially on non-Xperia or unsupported devices.

Step 1: Check Device Compatibility

• Ideally, install on a compatible Xperia model (Xperia series) that supports POBox Plus and meets the Android version requirement. The blog notes that newer devices sometimes face compatibility problems. 
• Ensure your Android version is sufficient (some versions note Android 14+ for latest builds).
• If your device isn't supported, the APK may fail to install or function properly.

Step 2: Enable Unknown Sources / Install Unknown Apps

• Since you’ll be installing an APK directly:
• Go to Settings → Security (or Settings → Apps & notifications → Special app access → Install unknown apps)
• Enable installation from the browser or file manager you will use.

Step 3: Download the APK

• Find a trustworthy source (e.g., APKRings shows POBox Plus overlay package versions) 
• Download a version compatible with your device — for example, version 3.9.A.0.1 or similar that supports your Android version.

Step 4: Install the APK

• Locate the downloaded APK in your file manager.
• Tap to install. If it fails due to incompatible device architecture or signature, you may need an older version. The blog suggests using SAI (Split APKs Installer) as an alternative. 
• After installation, go to Settings → System → Languages & Input → On-screen keyboard → Manage keyboards and enable POBox Plus. Then select it as your default keyboard.

Step 5: Configure the Keyboard

• Open a text input field and switch to POBox Plus keyboard.
• Configure input settings: prediction, conversion, kana/romaji layout etc (within POBox settings).
• Test if Japanese input runs smoothly (kana→kanji conversions, layout preferences etc).

Conclusion

POBox Plus APK represents the legacy and precision of Sony’s language technology.
It remains one of the most refined Japanese IMEs, delivering unmatched text prediction, elegant UX, and privacy-friendly offline functionality.
If you type frequently in Japanese or wish to bring back the Xperia keyboard experience, POBox Plus is an excellent tool — as long as you install it from a legitimate source and use it on a compatible device.