2009-10: Christian Djoos played for Brynas' U18 and U16 squads and competed for Gastrikland in the TV-Pucken high school tournament. In 39 games for Brynas U18 (including four playoff games), Djoos was a team-best plus-60 with 5 goals and 13 assists and had 70 penalty minutes. Brynas U18 won the gold medal. In six games with the U16 squad he scored 2 goals with 4 assists and was plus-three with 8 penalty minutes. In eight games for Gastrikland he scored 2 goals with 4 assists.
2010-11: Djoos appeared in 11 games for Brynas' U20 team and was the leading scorer amongst defensemen for Brynas' U18 team. Djoos had 1 assist and was minus-seven in limited ice time with Brynas U20. Playing with Brynas U18 he had 11 goals with 34 assists and led all juniors with a plus-56 plus/minus. Brynas U18 won a bronze medal. In five playoff games he had 5 assists and was plus-four with 4 penalty minutes.
2011-12: Djoos made his Elitserien debut – appearing in one game – in addition to playing for Brynas at the U20 and U18 levels and representing Sweden in the 2012 U18 World Junior Championship. Djoos had five minutes of ice time for league champion Brynas in his only appearance. Djoos scored 3 goals with 21 assists and was plus-11 with 22 penalty minutes in 40 games with Brynas U20 and was scoreless in two playoff games. He skated in 12 games with Brynas U18 (including five playoff games) and scored 6 goals with 8 assists. In six games at the U18 WJC he scored 1 goal with 2 assists and was plus-three with 2 penalty minutes. Sweden won a silver medal. Djoos was ranked 18th amongst European skaters in Central Scouting's final rankings prior to the 2012 NHL Draft.