Ivashka became the first Belarusian to win a trophy at ATP Tour level since Max Mirnyi in 2003 thanks to a crushing 6-0 6-2 victory.
The unheralded 27-year-old, who needed just 56 minutes to claim silverware, also became the eighth first-time winner on Tour this season and the sixth champion ranked outside the top 50 in 2021.
"This week, I had everything," Ivashka, who won 90 per cent of points on his first serve against Swedish opponent Ymer in the final of the ATP 250 event, said.
Ivashka, who stunned No.1 seed and 2016 champion Pablo Carreno Busta en route to glory, added: "In the second round, I was booking my flights to New York. I was two points away from losing and now I am standing here.
"It is incredible how things can change in one match. It was an unbelievable pleasure to play here. I feel amazing to play in front of such a nice crowd and I really enjoyed it."