diff --git a/Sources/FoundationEssentials/String/String+IO.swift b/Sources/FoundationEssentials/String/String+IO.swift
index 55f8e19ae..640efe422 100644
--- a/Sources/FoundationEssentials/String/String+IO.swift
+++ b/Sources/FoundationEssentials/String/String+IO.swift
@@ -24,6 +24,11 @@ dynamic public func _cfMakeStringFromBytes(_ bytes: UnsafeBufferPointer<UInt8>,
     // Provide swift-corelibs-foundation with an entry point to convert some bytes into a String
     return nil
 }
+
+dynamic package func _icuMakeStringFromBytes(_ bytes: UnsafeBufferPointer<UInt8>, encoding: String.Encoding) -> String? {
+    // Concrete implementation is provided by FoundationInternationalization.
+    return nil
+}
 #endif
 
 @available(macOS 10.10, iOS 8.0, watchOS 2.0, tvOS 9.0, *)
@@ -184,6 +189,17 @@ extension String {
                 }
             }
             self = bytes.withContiguousStorageIfAvailable(buildString) ?? Array(bytes).withUnsafeBufferPointer(buildString)
+        case .japaneseEUC:
+            // Here we catch encodings that are supported by Foundation Framework
+            // but are not supported by corelibs-foundation.
+            // We delegate conversion to ICU.
+            guard let string = (
+                bytes.withContiguousStorageIfAvailable({ _icuMakeStringFromBytes($0, encoding: encoding) }) ??
+                Array(bytes).withUnsafeBufferPointer({ _icuMakeStringFromBytes($0, encoding: encoding) })
+            ) else {
+                return nil
+            }
+            self = string
         #endif
         default:
 #if FOUNDATION_FRAMEWORK
diff --git a/Sources/FoundationEssentials/String/StringProtocol+Essentials.swift b/Sources/FoundationEssentials/String/StringProtocol+Essentials.swift
index 4c99c1599..1447385e0 100644
--- a/Sources/FoundationEssentials/String/StringProtocol+Essentials.swift
+++ b/Sources/FoundationEssentials/String/StringProtocol+Essentials.swift
@@ -91,6 +91,11 @@ dynamic public func _cfStringEncodingConvert(string: String, using encoding: UIn
     // Dynamically replaced by swift-corelibs-foundation to implement encodings that we do not have Swift replacements for, yet
     return nil
 }
+
+dynamic package func _icuStringEncodingConvert(string: String, using encoding: String.Encoding, allowLossyConversion: Bool) -> Data? {
+    // Concrete implementation is provided by FoundationInternationalization.
+    return nil
+}
 #endif
 
 @available(FoundationPreview 0.4, *)
@@ -249,6 +254,11 @@ extension String {
                     buffer.appendElement(value)
                 }
             }
+        case .japaneseEUC:
+            // Here we catch encodings that are supported by Foundation Framework
+            // but are not supported by corelibs-foundation.
+            // We delegate conversion to ICU.
+            return _icuStringEncodingConvert(string: self, using: encoding, allowLossyConversion: allowLossyConversion)
 #endif
         default:
 #if FOUNDATION_FRAMEWORK
diff --git a/Sources/FoundationInternationalization/ICU/ICU+StringConverter.swift b/Sources/FoundationInternationalization/ICU/ICU+StringConverter.swift
new file mode 100644
index 000000000..0ad69aecc
--- /dev/null
+++ b/Sources/FoundationInternationalization/ICU/ICU+StringConverter.swift
@@ -0,0 +1,204 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if canImport(FoundationEssentials)
+import FoundationEssentials
+#endif
+internal import _FoundationICU
+
+private extension String.Encoding {
+    var _icuConverterName: String? {
+        // TODO: Replace this with forthcoming(?) public property such as https://github.com/swiftlang/swift-foundation/pull/1243
+        // Note: UTF-* and US-ASCII are omitted here because they are supposed to be converted upstream.
+        switch self {
+        case .japaneseEUC: "EUC-JP"
+        case .isoLatin1: "ISO-8859-1"
+        case .shiftJIS: "Shift_JIS"
+        case .isoLatin2: "ISO-8859-2"
+        case .windowsCP1251: "windows-1251"
+        case .windowsCP1252: "windows-1252"
+        case .windowsCP1253: "windows-1253"
+        case .windowsCP1254: "windows-1254"
+        case .windowsCP1250: "windows-1250"
+        case .iso2022JP: "ISO-2022-JP"
+        case .macOSRoman: "macintosh"
+        default: nil
+        }
+    }
+}
+
+extension ICU {
+    final class StringConverter: @unchecked Sendable {
+        private let _converter: LockedState<OpaquePointer> // UConverter*
+
+        let encoding: String.Encoding
+
+        init?(encoding: String.Encoding) {
+            guard let convName = encoding._icuConverterName else {
+                return nil
+            }
+            var status: UErrorCode = U_ZERO_ERROR
+            guard let converter = ucnv_open(convName, &status), status.isSuccess else {
+                return nil
+            }
+            self._converter = LockedState(initialState: converter)
+            self.encoding = encoding
+        }
+
+        deinit {
+            _converter.withLock { ucnv_close($0) }
+        }
+    }
+}
+
+extension ICU.StringConverter {
+    func decode(data: Data) -> String? {
+        return _converter.withLock { converter in
+            defer {
+                ucnv_resetToUnicode(converter)
+            }
+
+            let srcLength = CInt(data.count)
+            let initCapacity = srcLength * CInt(ucnv_getMinCharSize(converter)) + 1
+            return _withResizingUCharBuffer(initialSize: initCapacity) { (dest, capacity, status) in
+                return data.withUnsafeBytes { src in
+                    ucnv_toUChars(
+                        converter,
+                        dest,
+                        capacity,
+                        src.baseAddress,
+                        srcLength,
+                        &status
+                    )
+                }
+            }
+        }
+    }
+
+    func encode(string: String, allowLossyConversion lossy: Bool) -> Data?  {
+        return _converter.withLock { (converter) -> Data? in
+            defer {
+                ucnv_resetFromUnicode(converter)
+            }
+
+            let utf16Rep = string.utf16
+            let uchars = UnsafeMutableBufferPointer<UChar>.allocate(capacity: utf16Rep.count)
+            _ = uchars.initialize(fromContentsOf: utf16Rep)
+            defer {
+                uchars.deallocate()
+            }
+
+            let srcLength = uchars.count
+            let capacity = srcLength * Int(ucnv_getMaxCharSize(converter)) + 1
+            let dest = UnsafeMutableRawPointer.allocate(
+                byteCount: capacity,
+                alignment: MemoryLayout<CChar>.alignment
+            )
+
+            var status: UErrorCode = U_ZERO_ERROR
+            if lossy {
+                var lossyChar: UChar = encoding == .ascii ? 0xFF : 0x3F
+                ucnv_setSubstString(
+                    converter,
+                    &lossyChar,
+                    1,
+                    &status
+                )
+                guard status.isSuccess else { return nil }
+
+                ucnv_setFromUCallBack(
+                    converter,
+                    UCNV_FROM_U_CALLBACK_SUBSTITUTE,
+                    nil, // newContext
+                    nil, // oldAction
+                    nil, // oldContext
+                    &status
+                )
+                guard status.isSuccess else { return nil }
+            } else {
+                ucnv_setFromUCallBack(
+                    converter,
+                    UCNV_FROM_U_CALLBACK_STOP,
+                    nil, // newContext
+                    nil, // oldAction
+                    nil, // oldContext
+                    &status
+                )
+                guard status.isSuccess else { return nil }
+            }
+
+            let actualLength = ucnv_fromUChars(
+                converter,
+                dest,
+                CInt(capacity),
+                uchars.baseAddress,
+                CInt(srcLength),
+                &status
+            )
+            guard status.isSuccess else { return nil }
+            return Data(
+                bytesNoCopy: dest,
+                count: Int(actualLength),
+                deallocator: .custom({ pointer, _ in pointer.deallocate() })
+            )
+        }
+    }
+}
+
+extension ICU.StringConverter {
+    private static let _converters: LockedState<[String.Encoding: ICU.StringConverter]> = .init(initialState: [:])
+
+    static func converter(for encoding: String.Encoding) -> ICU.StringConverter? {
+        return _converters.withLock {
+            if let converter = $0[encoding] {
+                return converter
+            }
+            if let converter = ICU.StringConverter(encoding: encoding) {
+                $0[encoding] = converter
+                return converter
+            }
+            return nil
+        }
+    }
+}
+
+
+@_dynamicReplacement(for: _icuMakeStringFromBytes(_:encoding:))
+func _icuMakeStringFromBytes_impl(_ bytes: UnsafeBufferPointer<UInt8>, encoding: String.Encoding) -> String? {
+    guard let converter = ICU.StringConverter.converter(for: encoding),
+          let pointer = bytes.baseAddress else {
+        return nil
+    }
+
+    // Since we want to avoid unnecessary copy here,
+    // `bytes` is converted to `UnsafeMutableRawPointer`
+    // because `Data(bytesNoCopy:count:deallocator:)` accepts only that type.
+    // This operation is still safe,
+    // as the pointer is just borrowed (not escaped, not mutated)
+    // in `ICU.StringConverter.decode(data:) -> String?`.
+    // In addition to that, `Data` is useful here
+    // because it is `Sendable` (and has CoW behavior).
+    let data =  Data(
+        bytesNoCopy: UnsafeMutableRawPointer(mutating: pointer),
+        count: bytes.count,
+        deallocator: .none
+    )
+    return converter.decode(data: data)
+}
+
+@_dynamicReplacement(for: _icuStringEncodingConvert(string:using:allowLossyConversion:))
+func _icuStringEncodingConvert_impl(string: String, using encoding: String.Encoding, allowLossyConversion: Bool) -> Data? {
+    guard let converter = ICU.StringConverter.converter(for: encoding) else {
+        return nil
+    }
+    return converter.encode(string: string, allowLossyConversion: allowLossyConversion)
+}
diff --git a/Tests/FoundationInternationalizationTests/StringTests+Data.swift b/Tests/FoundationInternationalizationTests/StringTests+Data.swift
new file mode 100644
index 000000000..7c0babc26
--- /dev/null
+++ b/Tests/FoundationInternationalizationTests/StringTests+Data.swift
@@ -0,0 +1,126 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+#if FOUNDATION_FRAMEWORK
+@testable import Foundation
+#else
+@testable import FoundationEssentials
+@testable import FoundationInternationalization
+#endif // FOUNDATION_FRAMEWORK
+
+#if canImport(TestSupport)
+import TestSupport
+#endif
+
+final class StringConverterTests: XCTestCase {
+    private func _test_roundTripConversion(
+        string: String,
+        data: Data,
+        encoding: String._Encoding,
+        file: StaticString = #filePath,
+        line: UInt = #line
+    ) {
+        XCTAssertEqual(
+            string.data(using: encoding), data, "Failed to convert string to data.",
+            file: file, line: line
+        )
+        XCTAssertEqual(
+            string, String(data: data, encoding: encoding), "Failed to convert data to string.",
+            file: file, line: line
+        )
+    }
+
+    func test_japaneseEUC() {
+        // Confirm that https://github.com/swiftlang/swift-foundation/issues/1016 is fixed.
+
+        // ASCII
+        _test_roundTripConversion(
+            string: "ABC",
+            data: Data([0x41, 0x42, 0x43]),
+            encoding: .japaneseEUC
+        )
+
+        // Plane 1 Row 1
+        _test_roundTripConversion(
+            string: "、。◇",
+            data: Data([
+                0xA1, 0xA2,
+                0xA1, 0xA3,
+                0xA1, 0xFE,
+            ]),
+            encoding: .japaneseEUC
+        )
+
+        // Plane 1 Row 4 (Hiragana)
+        _test_roundTripConversion(
+            string: "ひらがな",
+            data:  Data([
+                0xA4, 0xD2,
+                0xA4, 0xE9,
+                0xA4, 0xAC,
+                0xA4, 0xCA,
+            ]),
+            encoding: .japaneseEUC
+        )
+
+        // Plane 1 Row 5 (Katakana)
+        _test_roundTripConversion(
+            string: "ヴヵヶ",
+            data: Data([
+                0xA5, 0xF4,
+                0xA5, 0xF5,
+                0xA5, 0xF6,
+            ]),
+            encoding: .japaneseEUC
+        )
+
+        // Plane 1 Row 6 (Greek Alphabets)
+        _test_roundTripConversion(
+            string: "Σπ",
+            data: Data([
+                0xA6, 0xB2,
+                0xA6, 0xD0,
+            ]),
+            encoding: .japaneseEUC
+        )
+
+        // Basic Kanji
+        _test_roundTripConversion(
+            string: "日本",
+            data: Data([
+                0xC6, 0xFC,
+                0xCB, 0xDC,
+            ]),
+            encoding: .japaneseEUC
+        )
+
+        // Amendment by JIS83/JIS90
+        _test_roundTripConversion(
+            string: "扉⇔穴",
+            data: Data([
+                0xC8, 0xE2,
+                0xA2, 0xCE,
+                0xB7, 0xEA,
+            ]),
+            encoding: .japaneseEUC
+        )
+
+        // Unsupported characters
+        let sushi = "Sushi🍣"
+        XCTAssertNil(sushi.data(using: String._Encoding.japaneseEUC))
+        XCTAssertEqual(
+            sushi.data(using: String._Encoding.japaneseEUC, allowLossyConversion: true),
+            "Sushi?".data(using: .utf8)
+        )
+    }
+}
+