Background: Electronic health (eHealth) literacy would facilitate online information seeking and taking informed measures. Objective: We studied socioeconomic disparities in eHealth literacy and online COVID-19 information seeking, and their associations with COVID-19 preventive behaviors. Methods: The COVID-19 Health Information Survey (CoVHIns), using landline (n=500) and online surveys (n=1001), was conducted in adults in Hong Kong in April 2020. Chinese eHealth literacy scale (eHEALS, range 8¬–40) was used to measure eHealth literacy. COVID-19 preventive behaviors included wearing surgical masks, wearing fabric masks, washing hands, social distancing, and adding water/bleach to the household drainage system. Adjusted beta-coefficients and the slope indices of inequality (SII) for eHEALS score by socioeconomic status, adjusted odds ratios (aOR) for online COVID-19 information seeking by socioeconomic status, and aORs for high adherence to preventive behaviors by eHEALS score and online COVID-19 information seeking were calculated. Results: The mean score of eHEALS was 26.10 (standard deviation, 7.70). Age was inversely, but education and personal income were positively associated with eHEALS score and online COVID-19 information seeking (all P for trend <0.05). Participants who sought online COVID-19 information showed high adherence to wearing surgical mask (aOR 1.56 95% CI [1.15-2.13]), washing hand (aOR 1.33 [1.05-1.71]), social distancing (aOR 1.48 [1.14-1.93]), and adding water/bleach to household drainage system (aOR 1.67 [1.28-2.18]). Those with the highest eHEALS score was associated with high adherence to wearing surgical mask (aOR 3.84 [1.63-9.05]), washing hand (aOR 4.14 [2.46-6.96]), social distancing (aOR 2.25 [1.39-3.65]), and adding water/bleach to the household drainage system (aOR 1.94 [1.19-3.16]), compared those with the lowest eHEALS score. Conclusions: Chinese adults with higher socioeconomic status had higher eHealth literacy and online COVID-19 information seeking; both were associated with high adherence to the guideline on preventive behaviors during the COVID-19 pandemic.