diff --git a/stdlib/public/core/CMakeLists.txt b/stdlib/public/core/CMakeLists.txt
index ba69e14096a69..287b08cee96d8 100644
--- a/stdlib/public/core/CMakeLists.txt
+++ b/stdlib/public/core/CMakeLists.txt
@@ -132,6 +132,7 @@ set(SWIFTLIB_ESSENTIAL
   SetStorage.swift
   SetVariant.swift
   ShadowProtocols.swift
+  SharedString.swift
   Shims.swift
   Slice.swift
   SmallBuffer.swift
diff --git a/stdlib/public/core/GroupInfo.json b/stdlib/public/core/GroupInfo.json
index 380cdfe83e135..101a870b3301a 100644
--- a/stdlib/public/core/GroupInfo.json
+++ b/stdlib/public/core/GroupInfo.json
@@ -10,6 +10,7 @@
     "CharacterProperties.swift",
     "ICU.swift",
     "NormalizedCodeUnitIterator.swift",
+    "SharedString.swift",
     "SmallString.swift",
     "StaticString.swift",
     "String.swift",
diff --git a/stdlib/public/core/SharedString.swift b/stdlib/public/core/SharedString.swift
new file mode 100644
index 0000000000000..93a06d196b06c
--- /dev/null
+++ b/stdlib/public/core/SharedString.swift
@@ -0,0 +1,104 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2019 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
+//
+//===----------------------------------------------------------------------===//
+
+extension String {
+  /// Creates a `String` whose backing store is shared with the UTF-8 data
+  /// referenced by the given buffer pointer.
+  ///
+  /// The `owner` argument should manage the lifetime of the shared buffer and
+  /// its deinitializer is responsible for deallocating `buffer`. The `String`
+  /// instance created by this initializer retains `owner` so that deallocation
+  /// occurs after the string is no longer in use. The buffer _must not_ be
+  /// deallocated while there are any strings sharing it.
+  ///
+  /// This initializer does not try to repair ill-formed UTF-8 code unit
+  /// sequences. If any are found, the result of the initializer is `nil`.
+  ///
+  /// - Parameters:
+  ///   - buffer: An `UnsafeBufferPointer` containing the UTF-8 bytes that
+  ///     should be shared with the created `String`.
+  ///   - owner: An optional object that owns the memory referenced by `buffer`
+  ///     and is responsible for deallocating it.
+  public init?(
+    sharingContent buffer: UnsafeBufferPointer<UInt8>,
+    owner: AnyObject?
+  ) {
+    guard let baseAddress = buffer.baseAddress,
+      case .success(let extraInfo) = validateUTF8(buffer)
+    else {
+      return nil
+    }
+    let storage = __SharedStringStorage(
+      immortal: baseAddress,
+      countAndFlags: _StringObject.CountAndFlags(
+        sharedCount: buffer.count,
+        isASCII: extraInfo.isASCII))
+    storage._owner = owner
+    self.init(String(_StringGuts(storage)))
+  }
+
+  /// Creates a `String` whose backing store is shared with the UTF-8 data in
+  /// the given array.
+  ///
+  /// The new `String` instance shares ownership of the array's underlying
+  /// storage, guaranteeing that the `String` remains valid even if the `array`
+  /// is released.
+  ///
+  /// This initializer does not try to repair ill-formed UTF-8 code unit
+  /// sequences. If any are found, the result of the initializer is `nil`.
+  ///
+  /// - Parameter array: An `Array` containing the UTF-8 bytes that should be
+  ///   shared with the created `String`.
+  public init?(sharing array: [UInt8]) {
+    guard case (let owner, let rawPtr?) = array._cPointerArgs() else {
+      self = ""
+      return
+    }
+    let baseAddress = rawPtr.assumingMemoryBound(to: UInt8.self)
+    let buffer = UnsafeBufferPointer(start: baseAddress, count: array.count)
+    self.init(sharingContent: buffer, owner: owner)
+  }
+}
+
+extension Substring {
+  /// Calls the given closure, passing it a `String` whose contents are equal to
+  /// the substring but which shares the substring's storage instead of copying
+  /// it.
+  ///
+  /// The `String` value passed into the closure is only valid durings its
+  /// execution.
+  ///
+  /// - Parameter body: A closure with a `String` parameter that shares the
+  ///   storage of the substring. If `body` has a return value, that value is
+  ///   also used as the return value for the `withSharedString(_:)` method. The
+  ///   argument is valid only for the duration of the closure's execution.
+  /// - Returns: The return value, if any, of the `body` closure parameter.
+  public func withSharedString<Result>(
+    _ body: (String) throws -> Result
+  ) rethrows -> Result {
+    guard _wholeGuts.isFastUTF8 else {
+      // TODO: What's the right way to get the pointer for a non-fast-UTF-8
+      // string here?
+      return try body(String(self))
+    }
+
+    return try _wholeGuts.withFastUTF8 { utf8 in
+      let storage = __SharedStringStorage(
+        immortal: utf8.baseAddress! + self._offsetRange.lowerBound,
+        countAndFlags: _StringObject.CountAndFlags(
+          sharedCount: self._offsetRange.count,
+          isASCII: _wholeGuts.isASCII))
+      let str = String(_StringGuts(storage))
+      return try body(str)
+    }
+  }
+}
diff --git a/test/stdlib/SharedString.swift b/test/stdlib/SharedString.swift
new file mode 100644
index 0000000000000..560effa4feeb9
--- /dev/null
+++ b/test/stdlib/SharedString.swift
@@ -0,0 +1,90 @@
+// RUN: %target-run-simple-swift
+// REQUIRES: executable_test
+
+//
+// Tests for shared string APIs
+//
+
+import StdlibUnittest
+
+var SharedStringTests = TestSuite("SharedStringTests")
+
+func makeValidUTF8Buffer() -> UnsafeBufferPointer<UInt8> {
+  let ptr = UnsafeMutablePointer<UInt8>.allocate(capacity: 4)
+  ptr.initialize(repeating: UInt8(ascii: "a"), count: 4)
+  return UnsafeBufferPointer<UInt8>(start: ptr, count: 4)
+}
+
+func makeInvalidUTF8Buffer() -> UnsafeBufferPointer<UInt8> {
+  let ptr = UnsafeMutablePointer<UInt8>.allocate(capacity: 1)
+  ptr.pointee = 0x80  // orphaned continuation byte
+  return UnsafeBufferPointer<UInt8>(start: ptr, count: 1)
+}
+
+class BufferDeallocator {
+  let buffer: UnsafeBufferPointer<UInt8>
+
+  init(_ buffer: UnsafeBufferPointer<UInt8>) {
+    self.buffer = buffer
+  }
+
+  deinit {
+    buffer.deallocate()
+  }
+}
+
+SharedStringTests.test("String.init(sharingContent:owner:)") {
+  let buf = makeValidUTF8Buffer()
+  let str = String(sharingContent: buf, owner: BufferDeallocator(buf))
+
+  expectNotNil(str)
+  expectEqual("aaaa", str!)
+
+  // Show that the string didn't copy the buffer by modifying it in-place.
+  UnsafeMutableBufferPointer(mutating: buf)[0] = UInt8(ascii: "b")
+  expectEqual("baaa", str!)
+
+  // Show that mutating a copy works as expected.
+  var copy = str!
+  copy.append("c")
+  expectEqual("baaac", copy)
+  expectEqual("baaa", str!)
+}
+
+SharedStringTests.test("String.init(sharingContent:owner:) invalid UTF8") {
+  let buf = makeInvalidUTF8Buffer()
+  let str = String(sharingContent: buf, owner: BufferDeallocator(buf))
+
+  expectNil(str)
+}
+
+SharedStringTests.test("Substring.withSharedString(_:)") {
+  let original = "abcde"
+  let substr = original.dropFirst().dropLast()
+
+  substr.withSharedString { shared in
+    expectEqual("bcd", shared)
+  }
+}
+
+SharedStringTests.test("String.init(sharing:)") {
+  var array = [UInt8](repeating: UInt8(ascii: "a"), count: 4)
+  let str = String(sharing: array)
+
+  expectNotNil(str)
+  expectEqual("aaaa", str!)
+
+  // Show that mutating the array causes CoW and the original string isn't
+  // modified.
+  array[0] = UInt8(ascii: "b")
+  expectEqual("aaaa", str!)
+}
+
+SharedStringTests.test("String.init(sharing:) invalid UTF8") {
+  let array: [UInt8] = [0x80]  // orphaned continuation byte
+  let str = String(sharing: array)
+
+  expectNil(str)
+}
+
+runAllTests()