Kammala jumla mai zuwa: A ranar 13 ga Satumba 2018, an yi musayar Karlsson (tare da Francis Perron) zuwa San Jose Sharks a musayar Chris Tierney, Dylan DeMelo, Josh Norris, Rūdolfs Balcers, Sharks 'zaɓi na farko a 2020, zagaye na biyu a
2019 da zaɓin zagaye na biyu a 2021. Ya zira kwallaye na farko tare da Sharks a ranar 18 ga Nuwamba a nasarar 4 zuwa 0 a kan St. Louis Blues. Bayan jinkirin farawa zuwa kakar wasa, Karlsson ya zama mai tsaron baya na biyar a tarihin gasar don samun aƙalla taimako ɗaya a cikin wasanni 14 a jere bayan nasarar 7<unk>2 a kan Edmonton Oilers a ranar 8 ga Janairu 2019. A ranar 23 ga Disamba, an dakatar da Karlsson a karo na farko a cikin aikinsa na wasanni biyu don binciken da ba bisa ka'ida ba ga shugaban dan wasan Los Angeles Kings Austin Wagner. Raunin kashin baya ya sa Karlsson ya rasa wasanni da yawa a kusa da ƙarshen kakar kafin ya dawo don wasan karshe na kakar wasa da Colorado Avalanche. Zai gama kakar wasa tare da maki 45 a wasanni 53, mafi ƙarancin maki tun lokacin kakar 2012-13. Karlsson ya taimaka wa San Jose wajen kaiwa wasan karshe na Yammacin Turai a lokacin wasan karshe na 2019, duk da cewa ya sake samun rauni a wani wasa da St. Louis Blues, wanda hakan ya sa ya rasa wasan karshe na San Jose na wasan karshe lokacin da kungiyar ta yi rashin nasara 5<unk>1 a wasa na 6. Zai gama wasan kwaikwayo tare da maki 16 a wasanni 19. A ranar 31 ga Mayu, an yi masa tiyata.