Sergej Maslobojev; born 22 May 1987), nicknamed 'Kuvalda' ('Sledgehammer'), is a Lithuanian mixed martial artist, boxer and kickboxer, currently signed with GLORY, where he is the former Glory Light Heavyweight champion. He is ranked as the third best light heavyweight in the world by Combat Press as of September 2022, and third best by Beyond Kick as of October 2022.
 Maslobojev is the former two time King of Kings Heavyweight champion.
 Maslobojev saw kickboxing when he was a child, but only started training at the age of 18. He was impressed by Mirko Filipović, Wanderlei Silva and Remigijus Morkevičius.
 He is a professional Kickboxer and previously fought out of Sparta Fight gym in Lithuania, Vilnius city. In 2018 Sergej opened a brand new gym named Champion's House. He now has his own brand "Kuvalda Team". He regularly participates in various charity, being also a member of UNICEF. He undertakes training seminars for members of the Lithuanian Armed Forces.
 Maslobojev calls himself "a huge Dragonball Z nerd."
 Maslobojev made his professional debut against Steve Hooper in May 2007, when the two fought for the WKA Heavyweight Championship. The fight went into the tenth round, after which Sergej won by knockout. In his next fight, Maslobojev won the Baltic Muay Thai Light Heavyweight title, with a decision win over Kristaps Zile. After defeating Ricardo van den Bos, he entered the 2011 KOK European Heavyweight Grand Prix. He scored a first round knockout of Vasile Popovici in the quarterfinal and a third round knockout of Martynas Knyzelis in the semifinal, and faced Mindaugas Sakalauskas in the finals. Maslobojev beat Sakalauskas by knockout in the fifth round.
 Maslobojev went on a 2-2 run over his next four fights, before challenging the reigning WKN Heavyweight champion Vladimir Mineev at Fight Nights: Battle of Moscow 15. Mineev won the fight by majority decision.
 For his next fight, Maslbojev was scheduled to fight Lukasz Butkiewicz, at the KOK World GP 2014 in Gdańsk. He won the fight by a second-round knockout. He followed this up with a fight against Pavel Voronin, whom he beat by knockout as well. He would go on to win four of his next five fights, losing only to Roman Kryklia, before fighting Robert Dorin for the KOK Heavyweight Championship. Sergej won the fight by unanimous decision.
 After winning the KOK title, Maslobojev went on an eight fight, two year winning streak, during which he earned decision wins over Gokhan Gedik, Oleg Primachev and Thomas Bridgewater, as well as stoppage wins over Antonio Plazibat, Chaiyo Sitteacherjulun, Stanislav Voytenko, Carlos Ulberg and Bruno Susano. This run culminated in a title fight with Stéphane Susperregui for the WAKO World K-1 Heavyweight title. Maslobojev won by a fourth-round TKO, after Susperregui's corner was forced to throw in the towel. This winning streak was ended by Ondrej Hutnik at Yangames Fight Night 5, when the two of them fought for the WKN European Oriental Rules heavyweight title, with Hutnik winning a majority decision.
 In his next fight, Maslobojev was scheduled to fight Petros Vardakas. Maslobojev knocked Vardakas out in the second round.
 Following a TKO victory against Noureddine Ajnaou at KOK`55 World Series, Maslobojev signed with ONE Championship. He made his organizational debut at |ONE Championship: Grit and Glory, in a rematch with Antonio Plazibat. Maslobojev won the rematch by unanimous decision. In his second fight with ONE, he fought Florent Kaouachi at ONE Championship: Pursuit of Power, and won by unanimous decision.
 Moving away from ONE Championship, Maslobojev earned a unanimous decision against Rafael de Souza and a stoppage win against Fraser Weightman.
 In the second part of 2019, Maslobojev signed with GLORY. He was scheduled to make his debut against John King at Glory 68. King later withdrew, and Maslobojev was rescheduled to fight Bahram Rajabzadeh at Glory 69: Düsseldorf. He won the fight by unanimous decision.
 Maslobojev was next scheduled to fight a rematch with Fraser Weightman at Glory 73. The fight fell through, as Weightman was unable to gain a visa to fight in China. He was next set to face the K-1 heavyweight champion Roel Mannaart at Glory 75: Utrecht. Sergej won the fight by TKO, breaking Roel's jaw in the first round.
 Maslobojev was scheduled to fight Luis Tavares at Glory 78: Arnhem. He later withdrew from the bout with an injury.
 Maslobojev was scheduled to face Donegi Abena at Glory: Collision 3 on 23 October 2021. He won the fight by split decision.
 Maslobojev was scheduled to face Olivier Langlois-Ross at KoK Mega Battle on 20 November 2021. He won the fight by unanimous decision. 
 Maslobojev was booked to face Nika Kulumbegashvili at KOK 99 Mega Series on 19 March 2022. He won the fight by unanimous decision.
 Maslobojev was expected to face the #1 ranked Glory light heavyweight contender Luis Tavares for the vacant Glory Light Heavyweight Championship at Glory 81: Ben Saddik vs. Adegbuy 2 on 20 August 2022. The fight was cancelled on 10 August 2022, due to "reasons on the part of Tavares". Later that day, it was revealed that Tavares had tested positive for performance enhancing drugs. Maslobojev was instead scheduled to face Tarik Khbabez for the vacant title. Maslobojev won the fight by split decision.
 Maslobojev made his first title defense against the one-time Glory light heavyweight title challenger Donegi Abena at Glory 83 on February 11, 2023. He lost the fight by a fourth-round technical knockout. The ringside physician stopped the fight due to a cut on Maslobojev's right shin.