In a unit test you probably want to use [NSBundle bundleForClass:[self class]], and not [NSBundle mainBundle]. That's because the unit test is not a standalone app. With mainBundle you get something like /Applications/Xcode.app/Contents/Developer/Tools, but using bundleForClass gets you the bundle where your unit test class is located.

guard let pathString = Bundle(for: type(of: self)).path(forResource: "UnitTestData", ofType: "json") else {    fatalError("UnitTestData.json not found")}

In Swift

Swift 5 and above

guard let pathString = Bundle(for: type(of: self)).path(forResource: "UnitTestData", ofType: "json") else {    fatalError("UnitTestData.json not found")}guard let jsonString = try? String(contentsOfFile: pathString, encoding: .utf8) else {    fatalError("Unable to convert UnitTestData.json to String")}print("The JSON string is: \(jsonString)")guard let jsonData = jsonString.data(using: .utf8) else {    fatalError("Unable to convert UnitTestData.json to Data")}guard let jsonDictionary = try? JSONSerialization.jsonObject(with: jsonData, options: []) as? [String:Any] else {    fatalError("Unable to convert UnitTestData.json to JSON dictionary")}print("The JSON dictionary is: \(jsonDictionary)")

Swift 3 and 4

guard let pathString = Bundle(for: type(of: self)).path(forResource: "UnitTestData", ofType: "json") else {    fatalError("UnitTestData.json not found")}guard let jsonString = try? NSString(contentsOfFile: pathString, encoding: String.Encoding.utf8.rawValue) else {    fatalError("Unable to convert UnitTestData.json to String")}print("The JSON string is: \(jsonString)")guard let jsonData = jsonString.data(using: String.Encoding.utf8.rawValue) else {    fatalError("Unable to convert UnitTestData.json to NSData")}guard let jsonDictionary = try? JSONSerialization.jsonObject(with: jsonData, options: []) as? [String:AnyObject] else {    fatalError("Unable to convert UnitTestData.json to JSON dictionary")}print("The JSON dictionary is: \(jsonDictionary)")

Swift 2.2

    guard let pathString = NSBundle(forClass: self.dynamicType).pathForResource("UnitTestData", ofType: "json") else {        fatalError("UnitTestData.json not found")    }    guard let jsonString = try? NSString(contentsOfFile: pathString, encoding: NSUTF8StringEncoding) else {        fatalError("Unable to convert UnitTestData.json to String")    }    print("The JSON string is: \(jsonString)")    guard let jsonData = jsonString.dataUsingEncoding(NSUTF8StringEncoding) else {        fatalError("Unable to convert UnitTestData.json to NSData")    }    guard let jsonDictionary = try? NSJSONSerialization.JSONObjectWithData(jsonData, options: []) as? [String:AnyObject] else {        fatalError("Unable to convert UnitTestData.json to JSON dictionary")    }    print("The JSON dictionary is: \(jsonDictionary)")

*This incorporates Tom Harrington's answer which is in Objective C