Kalnietis started at point guard for the Lithuanian national basketball team that finished seventh in the 2006 FIBA World Championship. He averaged 6.0 points and 2.8 assists. He also played at the 2010 FIBA World Championship, as a starter, and won a bronze medal. He played in seven games for Lithuania at the 2012 Summer Olympics, averaging 5.5 points. He led the Lithuanian national team during EuroBasket 2013 to silver medals, averaging 12.1 points and 5 assists per game (the tournament's assist leader). He missed the 2014 FIBA Basketball World Cup, due to a clavicular dislocation, but returned to the national team at EuroBasket 2015. During the same championship, on 7 September, versus the Belgian national team, he achieved the EuroBasket's all-time single-game record of 13 assists, only to be tied days later for the same record by Miloš Teodosić. The Lithuanian national team won their second straight silver medal in 2015, and Kalnietis was once again the tournament's assists leader, with an average of 7.8 assists per game. Furthermore, he dished out 70 assists in total, which is the all-time record of assists for a single EuroBasket. Kalnietis also remained assists leader in EuroBasket 2017, averaging 7.2 per game.