This study examined novel coronavirus (COVID-19) messages on select Nigeria-based WhatsApp groups. Viewed through the lens of the Rumour Theory, the study applied content analysis and social semiotics (multimodal discourse analysis) methods. Data were elicited from three purposively selected WhatsApp groups, using the constructed and continuous weeks approach. The sample covered 6 weeks (42 days), spread across March, April, and May 2020. Findings from the content analysis showed that texts, comments, and linked message on COVID-19, had the highest frequency. The frequency of messages peaked in March and steeply tapered downwards in April and May 2020. The multimodal discourse analysis demonstrated a preponderance of messages about vaccines, treatment, prevention, lockdown, and conspiracy theories. A significant number of COVID-19 messages were based on rumours and misinformation from spurious sources, with a few from credible sources. The study recommended that to help flatten the misinformation curve, timely, unambiguous and accurate COVID-19 information should be provided from official sources.