In the cloud computing context, group data sharing, which is highly convenient for people who intend to share data with multiple members of a group, has attracted extensive research attention. However, the question of how data security might be ensured in this scenario, particularly as regards the security of group session keys, still remains an open one. To support secure group data sharing in cloud computing, it is essential to design a key agreement protocol characterized by both high flexibility and efficiency. Accordingly, this paper proposes a key agreement protocol based on Latin Square to reinforce the security of group data sharing. This protocol supports an arbitrary number of members to participate in a group and make equal contributions to generating a common session key among themselves. Compared with previous works, both the communication and the computational complexity in this protocol are significantly reduced. In addition, the services of authentication, key confirmation and fault tolerance are provided, enabling the protocol to resist different types of attacks. The results of both theoretical and experimental analysis indicate that the proposed protocol can efficiently support secure cloud data sharing for large-scale groups.