This paper proposes a secure and energy-efficient spatial data aggregation algorithm for sensor networks (SESDA for short). SESDA is an itinerary-based algorithm to achieve data aggregation. Owing to the well-designed itinerary for aggregate request propagation and data aggregation, SESDA is not susceptible to network topology and thus suitable for sensor networks with transient network topology, hence improves energy efficiency. In addition, to counter dramatic energy consumption caused by heavy encryption/ decryption operations, SESDA uses secure channel to obtain data privacy. SESDA needs no encryption/decryption operations during data aggregation, which significantly reduces the energy consumption, prolongs the lifetime of sensor networks, and achieves high accuracy of aggregation results due to small delivery delay. Theoretical analysis and experimental results show that SESDA has low traffic and energy consumption, high safety and accuracy.