Over the last decade, there has been an ever increasing number of citizens using online media to participate in and engage with politics. Social media sites and networks have enabled new opportunities for interactive and user-centered political experiences. Currently, there is a general scarcity of psychometrically validated and standardized instruments that assess politically-related constructs in the field of political sciences. The main aim was to develop a standardized psychometric tool to assess online political engagement among the general population that is valid and reliable. The psychometric properties of an Online Political Engagement Scale (OPEnS) were examined. The instrument assesses various online political actions people engage with during election campaigns. To develop the scale, data from the 2010 British Election Survey were used, and a total of 3,075 people participated in an online survey, post-election. The main findings obtained in the present study supported the undimensionality of the online political engagement construct given the results obtained from confirmatory factor analysis (CFA). The OPEnS appears to be a valid and reliable instrument for assessing this phenomenon, and may be useful in studies investigating newer patterns of online political engagement and disengagement. The next step will be a Portuguese validation of OPEnS.