diff --git a/Sources/PackageModelSyntax/AddTargetDependency.swift b/Sources/PackageModelSyntax/AddTargetDependency.swift
new file mode 100644
index 00000000000..fde0a5e69e6
--- /dev/null
+++ b/Sources/PackageModelSyntax/AddTargetDependency.swift
@@ -0,0 +1,103 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See http://swift.org/LICENSE.txt for license information
+// See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import Basics
+import PackageLoading
+import PackageModel
+import SwiftParser
+import SwiftSyntax
+import SwiftSyntaxBuilder
+
+/// Add a target dependency to a manifest's source code.
+public struct AddTargetDependency {
+    /// The set of argument labels that can occur after the "dependencies"
+    /// argument in the various target initializers.
+    ///
+    /// TODO: Could we generate this from the the PackageDescription module, so
+    /// we don't have keep it up-to-date manually?
+    private static let argumentLabelsAfterDependencies: Set<String> = [
+        "path",
+        "exclude",
+        "sources",
+        "resources",
+        "publicHeadersPath",
+        "packageAccess",
+        "cSettings",
+        "cxxSettings",
+        "swiftSettings",
+        "linkerSettings",
+        "plugins",
+    ]
+
+    /// Produce the set of source edits needed to add the given target
+    /// dependency to the given manifest file.
+    public static func addTargetDependency(
+        _ dependency: TargetDescription.Dependency,
+        targetName: String,
+        to manifest: SourceFileSyntax
+    ) throws -> PackageEditResult {
+        // Make sure we have a suitable tools version in the manifest.
+        try manifest.checkEditManifestToolsVersion()
+
+        guard let packageCall = manifest.findCall(calleeName: "Package") else {
+            throw ManifestEditError.cannotFindPackage
+        }
+
+        // Dig out the array of targets.
+        guard let targetsArgument = packageCall.findArgument(labeled: "targets"),
+              let targetArray = targetsArgument.expression.findArrayArgument() else {
+            throw ManifestEditError.cannotFindTargets
+        }
+
+        // Look for a call whose name is a string literal matching the
+        // requested target name.
+        func matchesTargetCall(call: FunctionCallExprSyntax) -> Bool {
+            guard let nameArgument = call.findArgument(labeled: "name") else {
+                return false
+            }
+
+            guard let stringLiteral = nameArgument.expression.as(StringLiteralExprSyntax.self),
+                let literalValue = stringLiteral.representedLiteralValue else {
+                return false
+            }
+
+            return literalValue == targetName
+        }
+
+        guard let targetCall = FunctionCallExprSyntax.findFirst(in: targetArray, matching: matchesTargetCall) else {
+            throw ManifestEditError.cannotFindTarget(targetName: targetName)
+        }
+
+        let newTargetCall = try addTargetDependencyLocal(
+            dependency, to: targetCall
+        )
+
+        return PackageEditResult(
+            manifestEdits: [
+                .replace(targetCall, with: newTargetCall.description)
+            ]
+        )
+    }
+
+    /// Implementation of adding a target dependency to an existing call.
+    static func addTargetDependencyLocal(
+        _ dependency: TargetDescription.Dependency,
+        to targetCall: FunctionCallExprSyntax
+    ) throws -> FunctionCallExprSyntax {
+        try targetCall.appendingToArrayArgument(
+            label: "dependencies",
+            trailingLabels: Self.argumentLabelsAfterDependencies,
+            newElement: dependency.asSyntax()
+        )
+    }
+}
+
diff --git a/Sources/PackageModelSyntax/CMakeLists.txt b/Sources/PackageModelSyntax/CMakeLists.txt
index cfab869efc1..c034d8d1705 100644
--- a/Sources/PackageModelSyntax/CMakeLists.txt
+++ b/Sources/PackageModelSyntax/CMakeLists.txt
@@ -10,6 +10,7 @@ add_library(PackageModelSyntax
   AddPackageDependency.swift
   AddProduct.swift
   AddTarget.swift
+  AddTargetDependency.swift
   ManifestEditError.swift
   ManifestSyntaxRepresentable.swift
   PackageDependency+Syntax.swift
diff --git a/Sources/PackageModelSyntax/ManifestEditError.swift b/Sources/PackageModelSyntax/ManifestEditError.swift
index cba8eb520dd..aaaf3351166 100644
--- a/Sources/PackageModelSyntax/ManifestEditError.swift
+++ b/Sources/PackageModelSyntax/ManifestEditError.swift
@@ -18,6 +18,8 @@ import SwiftSyntax
 /// package manifest programattically.
 package enum ManifestEditError: Error {
     case cannotFindPackage
+    case cannotFindTargets
+    case cannotFindTarget(targetName: String)
     case cannotFindArrayLiteralArgument(argumentName: String, node: Syntax)
     case oldManifest(ToolsVersion)
 }
@@ -33,6 +35,10 @@ extension ManifestEditError: CustomStringConvertible {
         switch self {
         case .cannotFindPackage:
             "invalid manifest: unable to find 'Package' declaration"
+        case .cannotFindTargets:
+            "unable to find package targets in manifest"
+        case .cannotFindTarget(targetName: let name):
+            "unable to find target named '\(name)' in package"
         case .cannotFindArrayLiteralArgument(argumentName: let name, node: _):
             "unable to find array literal for '\(name)' argument"
         case .oldManifest(let version):
diff --git a/Tests/PackageModelSyntaxTests/ManifestEditTests.swift b/Tests/PackageModelSyntaxTests/ManifestEditTests.swift
index 812b72bfe84..614ca912d3e 100644
--- a/Tests/PackageModelSyntaxTests/ManifestEditTests.swift
+++ b/Tests/PackageModelSyntaxTests/ManifestEditTests.swift
@@ -611,6 +611,46 @@ class ManifestEditTests: XCTestCase {
             )
         }
     }
+
+    func testAddTargetDependency() throws {
+        try assertManifestRefactor("""
+            // swift-tools-version: 5.5
+            let package = Package(
+                name: "packages",
+                dependencies: [
+                    .package(url: "https://github.com/apple/swift-testing.git", from: "0.8.0"),
+                ],
+                targets: [
+                    .testTarget(
+                        name: "MyTest"
+                    ),
+                ]
+            )
+            """,
+            expectedManifest: """
+            // swift-tools-version: 5.5
+            let package = Package(
+                name: "packages",
+                dependencies: [
+                    .package(url: "https://github.com/apple/swift-testing.git", from: "0.8.0"),
+                ],
+                targets: [
+                    .testTarget(
+                        name: "MyTest",
+                        dependencies: [
+                            .product(name: "Testing", package: "swift-testing"),
+                        ]
+                    ),
+                ]
+            )
+            """) { manifest in
+            try AddTargetDependency.addTargetDependency(
+                .product(name: "Testing", package: "swift-testing"),
+                targetName: "MyTest",
+                to: manifest
+            )
+        }
+    }
 }