Background: Informal caregivers of individuals with dementia have an increased risk to face social isolation due to progression of the disease. Online social media interventions might offer a new opportunity to increase access to social support and enhance positive interactions and openness in dementia care networks. Objective: This explorative pilot study describes (1) the development of an online social support intervention Inlife, and (2) the evaluation of the feasibility of this intervention and the measurements to assess its effectiveness. Methods: The Medical Research Council (MRC) framework guided the development of the online social support intervention. This is a stepwise approach that integrates potential users’ views with the development and validation of the program content. The program was developed by combining (1) individual caregiver interviews (n=10), (2) focus group sessions with experts and web designers (n=6), and (3) individual think-aloud tests (n=2). Subsequently, a pilot study with informal caregivers was conducted (n=25) to examine the program’s feasibility and preliminary effectiveness. Online self-report measures were completed at baseline and at four follow-up time points. Results: In total, 23 participants completed the newly developed Inlife intervention. Despite the high number of low-active users (17/23, 73%), Inlife had a good feasibility score of 7.1 (range: 1-10). The Calendar and Timeline were used most frequently and contributed to better care coordination and positive interactions. Conclusions: Although the Inlife platform received a sufficient feasibility rating, the uptake was not optimal. Therefore, the Inlife platform was adapted to limit the number of low-active users and improve user friendliness. Recommendations for additional treatment adherence were provided. The development according to the MRC framework and the sufficient feasibility rating of Inlife formed the basis for a future effectiveness study.