He previously played in the National Hockey League (NHL) for the Washington Capitals, Carolina Hurricanes and Montreal Canadiens.

Semin was trained in the Traktor Chelyabinsk hockey school and made his professional debut in Russia's second-tier Russian Major League in 2001–02.

Remaining in Russia for the subsequent season, he joined Lada Togliatti of the first-tier Russian Superleague (RSL) in 2002–03 and scored 10 goals and 7 assists in 47 games.

[1] Semin then reported to Maine to play for the American Hockey League (AHL)'s Portland Pirates, where he collected 15 points in 4 regular season and 7 playoff games.

During the 2004–05 NHL lockout, Semin returned to Russia and played 50 games for Lada Togliatti, recording 19 goals and 11 assists and compiling a plus-minus rating of +15.

However, other young Russian players, such as Nikolay Zherdev and Alexander Ovechkin, were allowed to return to their NHL clubs following the lockout, circumventing any additional military duty they had to serve in Russia.

[2] News about Lada's struggling financial situation early in the season initially offered the Capitals some hope that Semin would join the team after all.

It was reported the Russian team was expected to cut its payroll in half, while Semin was Lada's highest paid player.

With Semin's military obligations fulfilled, he agreed to a two-year contract with the Capitals on 11 April 2006, resolving the conflict between the two parties.

[8] Playing in his first game for the Capitals in two years, Semin scored the team's first goal of the 2006–07 season against the New York Rangers on 5 October 2006.

On 18 March 2007, Semin tallied the second natural hat-trick of his career against the Tampa Bay Lightning, scoring three goals on three successive shots in a span of 7:04 in the second period.

Late in the season, Capitals' head coach Glen Hanlon started Ovechkin and Semin together on the powerplay and occasionally on the team's first scoring line.

Along with Ovechkin and second-year forward Nicklas Bäckström, Semin helped form a highly offensive core with the Capitals in 2008–09.

Semin spent much of the first couple months of the season as the NHL's top point-scorer, but tailed off as he began to experience injury troubles.