Kalniņš qualified for the men's 100 m freestyle, as a member of the Latvian swimming team, at the 2012 Summer Olympics in London, by eclipsing a FINA B-standard time of 50.18 seconds from the European Junior Championships. He challenged seven other swimmers on the fourth heat, including British-born Paraguayan swimmer Benjamin Hockin. Kalniņš raced to second place, and dipped under a 50-second barrier by a quarter of a second (0.25) behind Turkey's Kemal Arda Gürdal, in his personal best of 49.96. Kalniņš failed to advance into the semifinals, as he placed thirtieth overall in the preliminaries.