The 25-year-old defenseman had five points (two goals, three assists) in 37 games during his rookie season with the Penguins last season, and could have become a restricted free agent July 1.
The Penguins were 22-10-5 with him in the lineup.
"Juuso has adjusted to the North American style of hockey seamlessly," Penguins assistant general manager Bill Guerin said. "He's a very confident guy, and he's also a great skater that moves the puck really well. We expect him to make an even bigger impact this year."
Riikola, who had two assists in five games with Wilkes-Barre/Scranton of the American Hockey League last season, had 24 points (eight goals, 16 assists) in 59 games with KalPa in Liiga, Finland's top professional league in 2017-18.
Undrafted, Riikola signed a one-year contract with Pittsburgh on May 18, 2018.
