Karl Johan Siward Mjällby (Swedish pronunciation: ; born 9 February 1971) is a Swedish football manager in charge of FC Stockholm in the Swedish fourth tier Division 2. A former player, he made his debut for the national team in 1997, and played 49 matches scoring 4 goals as a centre back. Mjällby was the team captain during the 2002 World Cup. He also took part in Euro 2000 and Euro 2004. He has been assistant manager of Bolton Wanderers, working alongside former Celtic team-mate and manager Neil Lennon.
Early on in Mjällby's sporting career he played both football and tennis and had the choice to turn professional in either. He made the decision to prioritise football and was signed by AIK in 1984 from IK Bele.
Mjällby spent the next fourteen years in AIK and established himself in the side with solid performances as a no-nonsense tackling midfield player. His performances at club level were noticed and he soon broke through to the Sweden national team. Although he was a squad member in 1992 when AIK won the league, he refused to accept the gold medal, stating that he felt he had not contributed enough. Therefore, it was a great day when lifelong AIK supporter Mjällby got his silverware in 1998 when he guided the club to a new league victory.