Royal Wahingdoh On 19 September 2011 Seityasen made his professional debut in the
Federation Cup against
Churchill Brothers for
Royal Wahingdoh who were then in the
I-League 2nd Division. The Royals created a major upset beating their fancied rivals 2–1 with Seityasen captaining Wahingdoh, and scoring the first goal. He was then an important part of Wahingdoh's
2011 I-League 2nd Division squad where they qualified for the
final round but failed to qualify to the I-League in the final round. He then participated in the
2012 I-League 2nd Division for Wahingdoh, where he scored twice in the group stages thus helping his team qualify to the
2012 I-League 2nd Division final round again. In the final round, it is only known that Seityasen appeared 8 times scoring thrice, but could not help his side gain promotion. In the
2013 I-League 2nd Division, he scored thrice but could not help his team qualify for the final round in a disappointing season. Finally, during the
2014 I-League 2nd Division, he scored three times to help his team qualify for the I-League. Seityasen played his first game of the new season in the
Federation Cup against
Mumbai in a 2–1 win. He would then make his
I-League debut against
Shillong Lajong and scored his first goal of the
2014-15 season from a
Jackichand Singh assist in a match they won by 2–1 scoreline. On 27 January 2015, he scored the winner against
Bharat FC in the 64th minute to give his side third win in a row. The young winger scored his first hat-trick in the I-League on 3 May 2015, when he netted thrice against
Salgaocar F.C. in a match played in Shillong.
Royal Wahingdoh won the tie 4–2.
NorthEast United (loan) In July 2015, Seityasen Singh was auctioned to play for
NorthEast United in the
2015 Indian Super League. Representing NorthEast United in the 2015 Indian Super League campaign, he was one of the 10 Indian players who were part of the auction. He picked up 4 Emerging Player of The Match Awards and received another National Team call-up for his excellent performances.
Salgaocar On 7 January 2016, Salgaocar confirmed that they have signed Royal Wahingdoh's star winger Seityasen Singh for 1 year. On 24 January, Seityasen made his league debut, coming-on as 38th-minute substitute for Alesh Sawant in an away match against Mumbai FC at the Cooperage Stadium.
DSK Shivajians On 1 January 2017,
DSK Shivajians confirmed that they have signed
NorthEast United's star winger Seityasen Singh sign for DSK Shivajians (loan).
NorthEast United On June 30, 2017, NorthEast retained Seityasen for third season of Hero ISL from end of loan from
DSK Shivajians.
Delhi Dynamos On 23 July 2017,
Delhi Dynamos FC confirmed that they have signed
NorthEast United's star winger Seityasen Singh.
NorthEast United On 1 July 2018,
NorthEast United confirmed that they have signed
Delhi Dynamos's winger Seityasen Singh.
Kerala Blasters FC On 17 August 2019,
Kerala Blasters confirmed that they have signed Seityasen Singh on a one-year deal. He made 10 appearances during the
2019-20 season of
Indian Super League and scored his first goal for the club against
Hyderabad FC. In September 2020, it was confirmed that Seityasen has extended his contract with the Blasters till 2022. Seityasen was included in the Kerala Blasters squad for the
2021 Durand Cup, where he appeared in all three matches for the club throughout the tournament. He played his first match of the 2021–22 Indian Super League season against defending champions Mumbai City FC on 19 November 2021 as a substitute for
Álvaro Vázquez in the 83rd minute of the game which they won 0–3 at full-time.
Hyderabad FC (loan) On 29 December, Kerala Blasters announced the loan departure of Seityasen to
Hyderabad FC for the rest of the season. ==International career==