In the era of open government, citizens are one of the stakeholders of a country. With the emerging concept of transparency and accountability, supported by ICTs deployments in government areas, citizens have more access to the government. Budget is among the many areas within governments that have utilized ICTs for many purposes, one of them is to share financial data. With this data, citizens can monitor how and where the government spent their taxes. The activity of following and monitoring the money flow is referred to as budget expenditure tracking. Citizens monitoring the flow of money through the website or downloading the raw spending data to the tracking can be seen as online engagement. Even though ICTs have been largely deployed, an online engagement still has its challenges. In some countries, the effort to provide opportunities for engagement to take place is relatively low. Governments need to provide opportunities for citizens to engage in the BET exercises process. Open spending portal is one of many ways to achieve this goal. Through this portal, governments can publish their spending data, and the public can freely access and reuse the data. However, the realization is not that simple. One of the reasons why online citizen engagement is difficult to realize is the unavailability of guidelines for e-participation. Therefore, there is a need to formulate clear guidelines on how to engage citizens in open spending portals for budget expenditure exercises. This research aims at developing a set of design principles that can enhance citizen engagement in open spending portals. Using Design Science Research, the author developed a set of design principles that can be grouped into two categories based on their purposes: to enable active citizens and to enable pro-active citizens. In total, there are 22 principles that have been developed to enhance citizen engagement in BET exercises through open spending portals. Since the principles are not yet implemented in existing designs, the evaluation was conducted to see the relevance between the developed design principles and existing problems. The potentiality of the developed design principles in enhancing citizen engagement was confirmed. However, future research needs to be done to assess the impact of the developed design principles in real life.