Background Employee turnover is increasing and in order for companies to retain their workforce, internal marketing (IM) needs to be applied. With internal communication (IC) being seen as the key to implementing employee oriented strategies and activities to motivate employees and coordinate cross-functional efforts within IM, internal social media (ISM), as known for fostering communication among employees, needs to be investigated regarding its influence on employee engagement. Purpose The purpose of this study was to investigate the psychological conditions of employee engagement on ISM and their relationship to employee engagement on ISM, moderated by the managerial communication styles assertive, aggressive and passive. Method An explanatory sequential mixed methods design has been applied, combining the advantages of both quantitative and qualitative data collection. Starting with 86 self-completion questionnaires, the findings, obtained by the application of a factor analysis in conjunction with multiple regression analysis, have been deepened with 5 semi-structured interviews with the help of a thematic analysis. Results and Conclusions The psychological conditions of employee engagement on ISM are indeed positively related to employee engagement on ISM. Furthermore, the assertive managerial communication style has shown to have a moderating effect on the relationship between the psychological conditions of employee engagement on ISM and employee engagement on ISM. Even though the moderating effect has been negative, the assertive communication style has been found to have a positive effect on employee engagement on ISM itself.