Fajdek claimed victory thanks to a world-leading throw of 81.98m, produced in round three, Polish state news agency PAP reported.
Olympic champion Nowicki came second with 81.03m, while Norway’s Eivind Henriksen scooped the bronze with 80.87m.
These three athletes also shared a podium, though in a different order, at last year's Tokyo Olympics.
For Fajdek, 33, it is the fifth consecutive world hammer title, having also won in Moscow (2013), Beijing (2015), London (2017) and Doha (2019).
Only the pole vault great Sergey Bubka has won more world championships in a single discipline.
Saturday’s result brings Poland’s medals tally at the World Athletics Championships Oregon22 to three: one gold and two silver.
(pm)
Source: PAP, worldathletics.org