Karl Johan Siward Mjällby (Swedish pronunciation: [?ju?an 'mj?l?b?]; 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.