Cyberstalking is a form of persecution that has proliferated with technology’s evolution. The present research aimed to develop a cyberstalking measure and observe its relations with Fear of Missing Out (FOMO), social media engagement, and sociodemographic variables. To achieve these goals, two studies were performed. In the first study, 200 subjects (76.5% female, with a mean age of 21.6 years) answered the 15 items originally developed for the scale. These data went trough exploratory factor analysis and Cronbach’s alpha to verify the reliability of the instrument. The results indicated the exclusion of five items, and after this removal, the scale was valid and reliable (α = 0.86). In the second study, which also had 200 subjects (65% female and an average age of 21.8 years), was realized confirmatory factor analysis (measuring the model fit), accompanied by correlations and mediation analysis. The analyzes demonstrated that the one-factor model was adequate (GFI = 0.98; CFI = 0.99; TLI = 0.99; RMSEA = 0.02; SRMR = 0.06). Path analysis showed social media engagement as a significant mediator of FOMO and gender’s impact on cyberstalking: Both had direct (FOMO: λ = 0.31; CI = 0.19–0.42; p < 0.01; Gender: λ = 0.12; CI = 0.02–0.22; p < 0.05) and indirect effects (FOMO: λ = 0.07; CI = 0.03–0.11; p < 0.01; Gender: λ = 0.04; CI = 0.01–0.07; p < 0.01).