He additionally played in the National Hockey League (NHL) with the Philadelphia Flyers, Tampa Bay Lightning and San Jose Sharks.

At the 2006 Winter Olympics, while playing for Team Finland, he was voted the Most Valuable Player (MVP) of the entire ice hockey tournament.

After two more seasons with TPS, Niittymäki moved to North America to join the Philadelphia Phantoms in the American Hockey League (AHL).

[3] In 2000, he won the Jarmo Wasama memorial trophy for Rookie of the Year,[4] and led TPS to their second straight SM-liiga championship.

[8] Even if he had opted to go immediately to the United States to play hockey, compulsory military service kept Niittymäki in Finland until January 2001.

[12] Niittymäki scored a game-winning, empty net, overtime, shorthanded, unassisted goal on April 11, 2004 versus the Hershey Bears.

[25][26][27][28] He continued to play backup goaltender for the Flyers for the next two seasons (07–08 and 08–09), setting a franchise record for most saves in a game in January 2008.

Niittymäki started his 2009–2010 season with great success; after his first ten games, he led the league in both save percentage (.939) and goals against average (1.93).

[14][37] On the first day of free agency in summer 2010, Niittymäki was signed to replace long time San Jose Sharks goaltender Evgeni Nabokov because of salary cap considerations.

[41] However, he sustained another lower body injury, resulting in him missing 17 games, during which Antti Niemi solidified his standing as the starting goaltender.

[48] Niittymäki was named to the Team Finland for the 2006 Winter Olympics after Kari Lehtonen and Miikka Kiprusoff had to pull out due to injuries.

[23] Despite being a relative unknown with little top-level experience, he put in a stellar performance and was named the MVP of the tournament after posting 3 shutouts in 6 games, leading Finland to the silver medal.

[24] Niittymäki played for Finland in the 2006 IIHF World Championships until the last game of the qualifying round against Canada, in which he got injured and was replaced by the team's reserve goalie Fredrik Norrena.