Behind Normal People, the second most-watched programme was Killing Eve, which debuted its third season in April.
The third season premiere of Killing Eve was the single most watched episode of any programme, amassing 7,398,000 streams in total.
The BBC also revealed that 2020 has seen 5.3 billion streams across all content on iPlayer, constituting a 33 per cent rise on last year and setting a new record for the broadcaster.
Episode one of Normal People, meanwhile, was watched 6,378,000 times.