Josi began with SC Bern in 2006. He was selected by the Nashville Predators in the second round, 38th overall, in the 2008 NHL Entry Draft. He was considered one of the top Swiss prospects in the draft.
He began the 2012–13 season with his youth team, SC Bern, because of the NHL lockout. He played alongside NHL stars John Tavares and Mark Streit. With the lockout resolved, Josi returned to play with the Predators and on 25 February 2013, Josi scored a career-high four points (two goals and two assists) in an overtime home win against the Dallas Stars.
Josi had a career high year during the 2014–15 season, ranking fifth among defensemen in scoring and finishing with a then-career high 55 points. At the end of the season Josi finished in the top five for the Norris Trophy vote.
During the 2015–16 season, Josi was selected to play in the 2016 NHL All-Star Game along with teammates Pekka Rinne, Shea Weber, and James Neal. He finished in the top five again for the Norris Trophy vote.
During the 2016–17 season, Josi was selected as an alternate captain along with James Neal. Josi’s defensive skill played a crucial role in the Predators 2017 Stanley Cup playoffs run. Despite being placed on long term injury reserve in January, he ended the season with 12 goals and 49 points.
Josi at the 2013 IIHF World Championship
|Representing Team Europe|
On October 29, 2019, Josi signed an eight-year, $72.472 million contract extension with the Predators. 
As the captain of the Predators, Josi has also been very active in the community. He has a partnership with the Best Buddies program in Nashville, giving tickets to each game for local kids with special needs. Josi also attends the Best Buddies prom and gala in Nashville with these kids. 
He won the James Norris Memorial Trophy in 2020, becoming the first Predators player and the first Swiss to win the award.
Josi was selected to play for Switzerland at the 2010 Winter Olympics. He previously represented Switzerland at the 2007 and 2008 IIHF World U18 Championships, the 2007, 2008 and 2009 IIHF World U20 Championship and the 2009 Ice Hockey World Championship. Josi was selected as the tournament MVP at the 2013 IIHF World Championship in Stockholm, Sweden.
Josi won a second silver medal with Switzerland at the 2018 IIHF World Championship in Denmark, falling to Sweden in the shootout.
Regular season and playoffs
|2005–06||SC Bern||SUI U20||5||0||0||0||0||—||—||—||—||—|
|2006–07||SC Bern||SUI U20||33||14||16||30||28||14||1||3||4||2|
|2006–07||SC Langenthal||SUI.2 U20||1||0||0||0||0||—||—||—||—||—|
|2007–08||Young-Sprinters Hockey Club||NLB||3||2||0||2||4||—||—||—||—||—|
|2007–08||SC Bern||SUI U20||—||—||—||—||—||6||2||2||4||14|
|2008–09||SC Bern||SUI U20||—||—||—||—||—||3||5||4||9||0|
- All statistics taken from NHL.com
Awards and achievements
|National League A champion||2010, 2013|
|NLA Media Swiss All-Star Team||2010|
|NHL All-Star Game||2016, 2019, 2020|
|James Norris Memorial Trophy||2020|
|NHL First All-Star Team||2020|
|IIHF World U18 Championship Top 3 Player on Team||2008|
|IIHF World Championship Silver medal||2013, 2018|
|IIHF World Championship Top 3 Player on Team||2013, 2014, 2015, 2019|
|IIHF World Championship MVP||2013|
|IIHF World Championship Best Defenceman||2013|
|IIHF World Championship All-Star Team||2013|
|IIHF World Championship Most Goals by Defenseman (4)||2013|
|IIHF World Championship Most Points by Defenseman (9)||2013|