The 24-year-old triumphed over Niculescu, currently the world number 55, 6-0, 2-6, 6-0 to take home $43,000 in prize money.
Arruabarrena, ranked 90th, earlier defeated Romania's Patricia Tig in the semi-finals.
It was Arruabarrena's second WTA singles title and the first since Bogota in February 2012.
She won doubles' titles at the Korea Open in 2014 and 2015.