Mother-to-child transmission is the predominant mode of transmission of HIV infection in children. Occasionally, horizontal transmission has also been reported. We report a child who was diagnosed HIV positive at the age of 3.5 years born to HIV-negative mother. Based on the available histories, no alternative modes of HIV transmission could be implicated. However, the paternal grandmother of the child was found to be HIV infected. To explore the likelihood that this HIV-infected relative was the most likely source from which this child acquired HIV infection postnatally, Bayesian phylodynamic analysis of the HIV of the child and parental grandmother was performed, which showed evidence of linkage of HIV transmission from an HIV-infected paternal grandmother to a child living with her through unknown route. Studies to identify modes of nonvertical HIV transmission may be useful in devising strategies to avert such nonvertical infections.