your json is not equal to your construct. this Json {"Synapses": [["ID: 0", "XPosition: 387.8125", "YPosition: 502.96875", "Width: 187.375", "Height: 122.0625"],["ID: 16", "XPosition: 490.859375", "YPosition: 947.15625", "Width: 49.28125", "Height: 69.6875"]]} is equal to the following construct struct NAME: Codable { let synapses: [[String]] enum CodingKeys: String, CodingKey { case synapses = "Synapses" } } in order to match your json to this construct // MARK: - Synapse struct Synapse: Codable { let id: Int let synapsX, synapsY, synapsWidth, synapsHeight: Double enum CodingKeys: String, CodingKey { case id = "ID" case synapsX = "SynapsX" case synapsY = "SynapsY" case synapsWidth = "SynapsWidth" case synapsHeight = "SynapsHeight" } } the json should be like { "Synapses": [ { "ID": 0, "SynapsX": 387.8125, "SynapsY": 502.96875, "SynapsWidth": 187.375, "SynapsHeight": 122.0625 }, { "ID": 16, "SynapsX": 490.859375, "SynapsY": 947.15625, "SynapsWidth": 49.28125, "SynapsHeight": 69.6875 } ] } Summery: your json did not match your construct from many point of views