Caregiver-mediated supports in general have shown mixed evidence for enhancing language outcomes in infants at higher likelihood of autism. While caregivers play a substantial role in caregiver-mediated supports, little is known about whether caregivers’ own subclinical autistic features – known as broader autism phenotype (BAP) – may moderate infant language outcomes. In secondary analysis of trial data, we examined whether caregiver BAP moderated the effectiveness of the iBASIS caregiver-mediated support program (received when infants were mean aged 12–18 months) for infant language outcomes (measured on parent-reported and direct assessment of receptive and expressive language). While lower caregiver BAP was linked to increased parent-reported infant vocabulary growth in general terms, it did not actually moderate the effect of the caregiver-mediated support program on those infant language outcomes. In relative terms therefore, infants of caregivers with both higher and lower BAP benefited equally from this support on parent-report compared to the comparison group. Caregiver BAP is associated with slower vocabulary growth in infants, but caregivers with autistic features can be recommended for this caregiver-mediated video-feedback based program, as their infants benefitted from such support.