Thelander began his career as a senior footballer in 2010 for Akademisk Boldklub in NordicBet Ligaen, the second-tier league in Danish football.
AaB
In the summer of 2012, Thelander signed a two-year contract with the Danish superliga club AaB. Rasmus played his first season in AaB primarily as a substitute. In the 2013–14 season he got his Superliga breakthrough, as defender Lasse Nielsen left for the Dutch team NEC Nijmegen, thus leaving a spot open in the central defense. Hereafter, Thelander would play each league game through the rest of the season and help his club win the first Danish double. Thelander scored two goals in the 4–2 victory against Copenhagen in the Danish cup final and won the Danish CupFighter of the Year .
Thelander signed a deal with the Greek team Panathinaikos on 2 July 2015. According to several Greek press sources he was offered a four-year contract having been recommended to Panathinaikos by René Henriksen, a former player with the club. Thelander had been courted in the previous season when he helped lead AaB win the double. Αfter a pre-season injury, he made his debut against Levadiakos on 23 September 2015. On 3 October 2015, during his third appearance with the club he scored his first goal in a 1–0 away win against Skoda Xanthi, with a header after a corner kick. On 26 July 2017, Thelander was the fifth footballer in eight months to file an appeal against Panathinaikos for payments' delay. In his appeal, he claimed an amount of €510,000 that covered the following year's contract as well. On 14 August 2017, the committee of the Greek Football Federation's financial disputes gave him the amount of €450,000.
On 17 May 2018, it was announced that Thelander would join Dutch side Vitesse on a three-year deal in July 2018. On 26 July, hemade his first appearance for Vitesse, starting in the first leg of their UEFA Europa League qualification round against Viitorul Constanța. On 23 August, news broke that Thelander had broken his ankle, keeping him off the field for some months. However, due to a quick recovery, he made his comeback in the first team on 3 November in a 1-0 loss against PSV Eindhoven. He made just 13 appearances in his sole season in Arnhem.
Return to AaB
On 22 August 2019, Thelander moved back to AaB on a three-year contract.
