The 21-year-old clocked a rapid time of 3:44.35 – the fastest in his career – to knock over a second and half off his best-ever time.
In a tight race, Wiffen finished just ahead of fellow Loughborough swimmer and training partner Felix Auboeck in second (3:44.71), and Danas Rapsys in third (3:46.56).
Wiffen previously set the Ireland record at the 2022 Commonwealth Games as he narrowly missed out on a podium place in Birmingham.
However, versus a highly competitive field in Sweden’s capital city, Wiffen secured gold and claimed a stunning new national record.