Online social networks (OSN) have started to integrate financial capabilities such as the usage of virtual currency. In OSNs with such capabilities, user accounts can also be used as financial accounts to manage virtual currency. Attackers are highly motivated to compromise user accounts and engage them in transactions to “steal” virtual currency. Such attacks represent a real and ongoing threat against a massive number of users in global social networks such as Tencent QQ. Aiming to address this emerging security concern, we have proposed an effective system, namely VC-Guard, to detect transactions that are likely initiated by attackers (i.e., suspicious transactions). Our system features the design and integration of multi-faceted features of each transaction. We have performed extensive experiments based on real-world data collected from Tencent QQ, a global leading OSN that has a virtual-currency-enabled ecosystem. Experimental results have demonstrated that our system has accomplished a high detection rate of 98.76% at a very low false positive rate of 1.24%.