Jertec has spent the majority of his career in his native Croatia, playing 63 times in the first division and another 49 times in the second division.
Early on in his career, he spent two seasons at Dinamo Zagreb starting in 2006 and then later another two seasons at Hajduk Split starting in 2008.
Most recently he joined NK Varazdin in 2016 and helped them gain promotion to the first division.
Jertec also played for Croatia's under-21 side eight times.
The attacking midfielder has become the club's fourth international signing, joining Alessandro Diamanti, Panagiotis Kone and Filip Kurto.
“We believe Dario will bring added quality, experience and leadership to our football team,” said United boss Mark Rudan.
“We look forward to seeing Dario challenge himself and his teammates as we create competition for places across the park.”