BACKGROUND: Penta3 coverage in Nigeria was low at 33 % in 2017. The most reported reason for non-vaccination was lack of knowledge about the immunization place, time, and need. To address knowledge gaps and improve vaccination uptake, we designed an Immunization Reminder and Information SMS System (IRISS) to educate and remind parents/caregivers about immunization using SMS. A formative study was conducted to understand the contextual and behavioural factors that would inform the IRISS intervention design and implementation. METHODS: We conducted the study in four Local Government Areas (LGAs) of Kebbi State Nigeria in October 2018, amongst a diverse selection of participants. Data on social norms about vaccinations, barriers to immunization uptake, mobile phone use, SMS message testing, and willingness to accept SMS reminders were collected from focus group discussions (N = 11), in-depth interviews (N = 12), and key informant interviews (N = 13). In addition, we assessed 33 messages covering schedule reminders, normative, motivational, educational, and informative contents for clarity, comprehensibility, relevance, cultural appropriateness, and ability to motivate action among community members from Argungu and Fakai LGAs. All interviews were analyzed using a thematic analysis approach. RESULTS: We interviewed 135 people, and 90 % were community members. While we found positive perceptions about immunizations among those interviewed, pockets of misconceptions existed among community members. Lack of awareness on the importance of vaccination was a consistent reason for under-vaccination across the LGAs. In addition, most community members do not own phones, could not read SMS messages, and were unaware of how to check/open text messages received. Despite concerns about low literacy levels and phone ownership, community members still saw a role in SMS reminders when phone owners receive messages. For instance, community leaders can disseminate said messages to community members through existing channels such as town announcers and religious gatherings. Therefore, the SMS becomes a source of information, with phone owners acting as a conduit to community dissemination mechanisms. We generally found the tested messages to be relevant, motivating, and culturally acceptable. CONCLUSIONS: SMS reminders have the potential to bridge the information gap in community awareness for vaccination, which can translate to improved immunization uptake. In rural communities with low literacy levels and phone ownership, immunization information can be disseminated when existing community leadership structures are engaged.