However, Ružička chose to move to North America and play with the Sarnia Sting and Sudbury Wolves of the Ontario Hockey League (OHL).
After being taken in the 2017 NHL Entry Draft 109th overall by the Calgary Flames, he developed within their system, playing in the American Hockey League (AHL) for the Stockton Heat.
[3] Growing up in Slovakia, Ružička trained at various hockey schools including Svišťov run by Adriana Hostůovecká.
[5] In order to continue playing hockey, Ružička and his mother lived in Pardubice for two years and he trained with an English tutor to improve his language skills.
[6] The Sarnia Sting of the Ontario Hockey League (OHL) also selected Ružička in the second round of the 2016 CHL Import Draft.
[7] He eventually chose to move to North America due to the disadvantage he experienced as a foreign player in the Czech Republic.
[8] He was also chosen to represent Team Cherry at the 2017 CHL/NHL Top Prospects Game as a 2017 NHL Entry Draft eligible player.
[9] In January 2017, Ružička was ranked 27th among all eligible North American skaters by the NHL Central Scouting Bureau (CSS).
Following the draft, Flames' scout Fred Parker praised his skills but acknowledged that he still struggled to figure out the North American game.
[20] Ružička spent his entire first professional season with the Calgary Flames' American Hockey League (AHL) affiliate, the Stockton Heat, and posted 27 points (10 goals, 17 assists) in 54 games.
[21] Due to the ongoing COVID-19 pandemic, the Heat played the 2020–21 season in Calgary as part of a newly formed Canadian Division.
[33] On 29 May 2024, Ružička was signed to a one-year contract by Spartak Moscow of the Kontinental Hockey League (KHL), playing alongside his brother-in-law.