<p>Seagrasses perform various critical functions within social–ecological systems, though populations have declined drastically. Restoring seagrass is crucial, requiring holistic consideration of ecological and human dimensions. To date, biophysical factors predominantly guide restoration and marine spatial planning (MSP). Sociocultural data underpin optimal and just restoration design, but remain a “missing layer” in MSP. To address this gap, we combined participatory mapping and ground-truthed remote sensing to map seagrass distribution, human activities, and place values in Sanday, UK. Local knowledge guided in situ surveys, co-producing Sanday’s first scientific seagrass records. We “found” two meadows absent from current seagrass modelling, mostly within protected areas, and highlighted corresponding human activities (notably shellfisheries) and values (humanistic and scientific). We propose three restoration priority sites, given seagrass extent, habitat protection, and compatible activities and values. This study demonstrates potential for transdisciplinary mapping to improve restoration planning, addressing the missing sociocultural layer and key gaps within MSP processes.</p>