It was the 73rd-ranked Riske's first tournament win since recovering from foot injuries last year that kept her out of competition for nine months.
The American, who was ranked 18th two years ago, improved to 3-8 in career finals, following earlier triumphs in 's-Hertogenbosch in 2019 and Tianjin in 2014.
It was the second final of the season for Riske, after losing to Jasmine Paolini of Italy in Portoroz, Slovenia, in September.
The 100th-ranked Cristian made it to her first final as a "lucky loser."
She lost in qualifying last Sunday but still entered the main draw after fellow Romanian Sorana Cirstea withdrew from the event with a calf injury.
Cristian further benefited on her way to the final from two opponents withdrawing with injuries: Rebecca Peterson of Sweden quit their last-16 match during the first set, and Simona Halep pulled out before their semifinal.
Cristian and Riske swapped breaks early in the opening set of the final before the Romanian gained control with dominating groundstrokes to win four straight games from 2-2.
Riske avoided break points and had nine winners in the second set to level the match.
The American was a break up in the decider but lost her advantage in a nine-minute service game at 4-3, and missed four break chances in the next.
She broke Cristian at 5-5 and closed out the win when the Romanian hit a backhand long.