Poland's Kajetanowicz and co-driver Maciej Szczepaniak scooped the title thanks to a second-placed finish at Rally Japan on Sunday, Polish state news agency PAP reported.
They had won three of this year's rallies, Mexico, Sardinia and Safari, according to news outlets.
Driving a Škoda Fabia RS Rally2, Kajetanowicz and Szczepaniak became the inaugural champions of the WRC2 Challenger series for private teams, the PAP news agency reported.
Kajetanowicz, a three-time European champion, afterwards thanked his co-driver, Poland's Orlen Rally Team, sponsors, coaches, fans and family.
"We can say we are world champions, and it's an amazing feeling," he said in Japan.
Source: PAP, interia.pl, Eurosport, Fédération Internationale de l'Automobile (FIA)