Sekelj was born in Spišská Sobota, which was then in the Austro-Hungarian Empire. It is now called Poprad, in Slovakia. His father was a veterinarian (animal doctor), and the family moved around extensively. They moved to Cheney, now in Romania. In 1922 they moved to Kikinda, Serbia. He went to elementary school in Kikinda. The family moved to Nikšić, in Montenegro where he finished high school. He went to Zagreb where he studied in law at university.