Valentijn was trained at the Chris Dolman gym, where he specialized in kickboxing and sambo before debuting at the Japanese professional wrestling promotionFighting Network RINGS. In 1999, Overeem participated in the RINGS King of Kings tournament, but he was submitted and eliminated by Antônio Rodrigo Nogueira at the first round. He would perform better at the next edition of the tournament, advancing on the first round after eliminating sambo champion Suren Balachinskiy. Overeem earned a yellow card for holding the ropes to avoid a takedown at one point, but he made up by outstriking Suren and focusing on kicking his injured leg, finishing the match by knee strikes. Valentijn then advanced further against famed luta livre practitioner Renato Sobral in an upset. Overeem was the superior striker, prompting Sobral to take him down. Overeem looked for a Kimura lock, the Brazilian reversed it into an armbar creating an exchange that ended with Overeem taking by accident an illegal punch to the jaw that almost knocked him out. As Overeem seemed unable to continue, both judges and the referee proposed to stop the match, but Valentijn's corner insisted on continuing. Returning to the fight, Overeem managed to compose himself and take down Babalu by kosoto gake, which the lutador countered by threatening with another armbar from the bottom. This time, however, Overeem captured a leg and quickly transitioned into a toehold, making Sobral tap out. After eliminating Yoshihisa Yamamoto by armbar in under one minute, Overeem continued advancing, and got another upset by choking out Randy Couture in a failed takedown. The final match of the tournament was against Antônio Rodrigo Nogueira, with Overeem submitted once again in the first round.
Strikeforce
On 9 June 2010, Overeem signed to fight under the Strikeforce banner. He was scheduled to fight Antônio Silva on 4 December 2010, but suffered an injury that caused him to withdraw from the fight. He eventually made his promotional debut against Ray Sefo at on 12 February 2011 where he won in the first round by using a neck crank. Overeem then returned against Chad Griggs at Strikeforce: Overeem vs. Werdum on 18 June 2011, in Dallas, Texas. Overeem lost as he tapped out due to punches. On 3 August 2011, Golden Glory announced that Overeem had been released from his contract by Strikeforce.
Post-Strikeforce
On 25 February, Overeem won his fight against Polish kickboxer and mixed martial artist Marcin Różalski at KSW 18 in Płock, Poland. He replaced injured Jérôme Le Banner and took the fight on 2 days notice. Overeem won by submission in the first round. Overeem fought in a kickboxing match on 23 February 2013 after 16 years of inactivity as a professional kickboxer where he faced James Wilson, a former professional boxer turned kickboxer. The fight was held in Miami on the debut event of S-1 Challenge, it ended in a No Contest decision due to an unsafe ring as both fighters fell over the ropes with Wilson falling twice. On 7 December 2013, Overeem fought Mikhail Mokhnatkin at Fight Nights: Battle of Moscow 14. Overeem lost via KO in the first round. Overeem faced Konstantin Gluhov in a rematch on 12 April 2014 at PFC 6: Pancrase Fighting Championship 6. Overeem lost by first-round KO. Overeem would then lose his next bout against Ante Delija via TKO at HOG: House of Gladiators 9 in round one. In 2013, Overeem featured in the video clip Street Credibility by the Belgian rapper CHG Unfadable.
