Partway through his four-year junior career with TPS in the junior SM-liiga, Niittymäki was selected by the Flyers 168th overall in the 1998 NHL Entry Draft. He continued to play in Finland with TPS for his first year out of the junior leagues, and he won the Finnish elite league's Rookie of the Year trophy. After two more seasons with TPS, Niittymäki moved to North America to join the Philadelphia Phantoms in the American Hockey League (AHL). When the Phantoms won the AHL Calder Cup championship, he was named the MVP of the playoffs. He then played four seasons for the Philadelphia Flyers before signing as a free agent with the Tampa Bay Lightning. One year later, he signed as a free agent with the San Jose Sharks, but he did not play many games for them over the next two years due to hip injuries. He played his final season with his youth team TPS, and he announced his retirement in 2013.